KABUL, Afghanistan — United States troops and their NATO allies intend to be out of Afghanistan by early to mid-July, well ahead of President Biden’s Sept. 11 withdrawal deadline, military officials said, in what has turned into an accelerated ending to America’s longest war.

But the race to the exits, which has picked up steam as planeloads of equipment and troops are flown out of the country, leaves the United States grappling with huge unresolved issues that officials had thought they would have more time to figure out.

The Pentagon still has not determined how it will combat terrorist threats like Al Qaeda from afar after American troops leave. Nor have top Defense Department officials secured agreement from allies about repositioning American troops in other nearby countries. And administration officials are still grappling with the thorny question of whether American warplanes — most likely armed Reaper drones — will provide air support to Afghan forces to help prevent the country’s cities from falling to the Taliban.

“Withdrawing forces is actually a really delicate kind of operation that has risks associated with it,” said Michèle A. Flournoy, a former under secretary for defense under President Barack Obama. “There’s a lot they have to work through before the last person steps on the plane — especially when you have allies on the ground who are going to inherit what we are leaving behind.”

Mr. Biden announced last month that all forces would be out by Sept. 11, overruling his military advisers, who wanted to keep a residual American troop presence in the country to help Afghan security forces hold back the Taliban from key population centers along with counterterrorism missions. Almost immediately after Mr. Biden’s announcement, Pentagon officials began taking steps to make sure this interim period — one official called it “purgatory” between the announcement and the completion of the withdrawal — was as short as possible.

The Pentagon wanted to avoid what officials said could be a nightmare scenario: a combat-related death in Afghanistan after the president had announced that American troops were withdrawing. Such a loss could prompt a public outcry over why American troops were being put at risk for a lost cause, officials said. That, and Mr. Biden had extended the American presence beyond May 1, the proposed troop withdrawal date outlined in the U.S.-Taliban peace deal last year.

In addition, once the decision was made to leave, officials discovered that there was not that much physically left to move. The Obama and Trump administrations had already cut the American troop presence back to around 3,500 from more than 100,000 in 2011.

Military officials quickly realized that they could be out by early to mid-July; NATO and allied forces are aiming to meet that deadline as well, officials said. The German military, which maintains a modest contingent of troops in Afghanistan’s north, is struggling to meet the U.S. schedule and is racing to catch up, according to U.S. officials.

Kandahar Airfield, once one of the largest U.S. bases in Afghanistan, was quietly shuttered this month along with several other smaller bases. And in the coming days U.S. fighter jets, lethal fixtures in the country since the start of the war, will begin departing from the sprawling Bagram Air Base for good, the officials added. Bagram, the largest U.S. base in the country, is the main hub for the withdrawal and will most likely be the last base the United States leaves behind.

The rapid withdrawal has exposed a variety of complex problems that have yet to be resolved and are provoking intense concern.

Officials have yet to decide how to ensure security for Kabul’s international airport, an issue that could determine whether other nations can maintain a diplomatic presence in Afghanistan. Australia announced on Tuesday that it is shutting down its embassy in Kabul until the security situation in the country improves.

Around 17,000 private contractors — more than 6,000 of them U.S. citizens — are expected to leave along with U.S. and allied military forces, potentially leaving Afghanistan’s military, and especially its air force, without vital support.

This story is based on interviews with more than a dozen American, European and Afghan officials, some of whom spoke on the condition of anonymity to discuss military planning.

Publicly, administration officials are insisting that the Afghan government can still hang on after American troops leave.

“It’s not a foregone conclusion, in my professional military estimate, that the Taliban automatically win and Kabul falls,” Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, told reporters this month. “There’s a significant military capability in the Afghan government. And we have to see how this plays out.”

But there are far more questions than answers about what comes next for the conflict-ridden country as any hope of peace remains fleeting and the Taliban continue to seize territory from beleaguered government forces.

Officials are scrambling to improvise solutions that senior officials concede may be temporary or makeshift, and likely very costly, such as shuttling dozens of armed Reaper drones more than 1,000 miles each way between the Persian Gulf and Afghanistan to support Afghan forces.

NATO will most likely continue training Afghan special operations forces — one of Afghan security forces’ most important units — outside the country. A western official with knowledge of the mission said one possible training location could be Jordan.

American military officials have discussed keeping troops in neighboring countries to maintain a reaction force to go after terrorist threats that might arise in Afghanistan from groups such as Al Qaeda or the Islamic State.

But reaching agreements, especially with some of the former Soviet republics bordering Afghanistan’s north, remains unlikely given the proximity of those countries to Russia and the Kremlin’s influence, according to U.S. officials.

Pakistan might be a possibility, but given the country’s longstanding support to the Taliban and often tense relationship with the United States, there is little hope in Washington that U.S. forces could be stationed there, at least ones that would be acknowledged publicly.

To keep tabs on the military situation on the ground, the U.S. military wants to continue using some version of what it calls the Combined Situational Awareness Room, where it coordinates with its Afghan counterparts (often over WhatsApp), funneling information and helping put air support and other forces into place on the battlefield. But it remains unclear where the command center would be, with options including the American Embassy or outside the country.

Though the Afghan Air Force has become increasingly capable in recent years, American drones and other surveillance aircraft still provide key targeting information. And U.S. strikes, though reduced under extremely restrictive rules of engagement, still occur as international forces depart and Afghan security forces struggle to hold ground.

U.S. military officials believe the United States will devote a significant number of reconnaissance aircraft to continue to help the Afghan forces but will limit airstrikes to “counterterrorism operations” only, a loose description that has been used in the past to justify a variety of actions.

With no bases to position aircraft close to Afghanistan, that means American aircraft will have to fly from bases in the Middle East or from aircraft carriers in the Arabian Sea to support Afghan forces or to conduct counterterrorism missions from “over the horizon.”

For prop-powered surveillance drones and planes, that means several-hour trips just to get to Afghanistan.

For jets based on aircraft carriers, that means frequent midair refueling stops. As land-based U.S. jets leave Afghanistan, United States forces are struggling to meet the demand for carrier-based aircraft because of an increased need for refueling tankers. For now, the jets onboard the U.S.S. Eisenhower in the Arabian Sea can fulfill only around 75 percent of the requests over Afghanistan, a military official said.

Questioned by lawmakers last month about the challenges of countering terrorist threats in Afghanistan after American troops leave, Gen. Kenneth F. McKenzie Jr., the head of the Pentagon’s Central Command, said, “It’s going to be extremely difficult to do, but it is not impossible.”

Thousands of private contractors are streaming out of Afghanistan alongside U.S. and allied forces.

A U.S. Army document outlining the contractor withdrawal, dated April 20, called the move “G2Z,” or “Go to Zero.” “Nobody wants to have a contractor left behind or be the company that is highlighted on ‘60 Minutes’ as the company that lost employees in Afghanistan,” the memo said.

But as U.S. officials confront what the future of Afghanistan will look like without a foreign military presence, it has become readily apparent that the Afghan military is not ready to be without private contractors, especially for its air force, which relies almost completely on them for maintenance.

On May 14, an Afghan Air Force helicopter was damaged on a resupply mission in Helmand Province when a Taliban mortar damaged its stabilizer. In the past, the part would have been switched out and repaired within a day, the helicopter’s pilot said, as contractors used to be present at the main Afghan base in the southern province.

Now, with the contractor mechanics back in Kabul and apparently leaving soon, the helicopter is still grounded and the part has yet to make its way south. One Afghan Air Force contractor described the entire situation as chaos.

Afghan officials are considering hiring contractors independently, and American commanders are wrestling with a variety of options, including coaching lesser-trained Afghan mechanics over video conference calls.

Then there is the issue of Kabul’s international airport, which serves both civilian and military aircraft. Currently, several hundred troops from Turkey — a NATO member — are defending the airport, but it is unclear whether they will remain, raising fears in the diplomatic community about getting in and out of the country safely.

At Kandahar airfield in the south, another split civilian-military facility, the pains of the withdrawal have already been felt. Since this winter, when the airport was handed over to the Afghans, a main radar at the airport broke and now has been disabled. Afghan officials are looking for contractors to operate it, but until then, airline flights have been reduced and can land only during the day.

U.S. officials believe the Turks are looking for concessions for them to remain at the airport in Kabul; what those are remains unclear, and military relations between Turkey and the United States are strained. President Donald J. Trump kicked Turkey out of the F-35 stealth fighter jet program in 2019 in retaliation for Turkey’s purchase of Russian S-400 surface-to-air missile systems.

But for the embassies of the United States and other Western allies to remain secure in Kabul, the West needs Turkish forces at the airport, so some horse-trading is looming, officials said.

Thomas Gibbons-Neff reported from Kabul, and Eric Schmitt and Helene Cooper from Washington. Jim Huylebroek and Najim Rahim contributed reporting from Kabul, and Taimoor Shah from Kandahar, Afghanistan.