WASHINGTON — The top American commander in the Middle East said on Tuesday that it would be “extremely difficult” for the United States to watch and counter terrorist threats in Afghanistan like Al Qaeda after American troops leave the country by Sept. 11.

The head of the U.S. Central Command, Gen. Kenneth F. McKenzie Jr., offered the first extensive comments by a top commander about the effect of President Biden’s decision to withdraw more than 2,500 American troops from Afghanistan. Mr. Biden rejected the advice of top Pentagon and military advisers to keep a small force in place.

Among the major challenges once troops have left will be how to track and potentially attack militant groups in Afghanistan, a landlocked nation far from any major American base. General McKenzie said the administration was discussing with other countries where it could reposition forces to prevent Afghanistan from again becoming a terrorist base.

Possibilities in the region include Tajikistan, Kazakhstan and Uzbekistan, but those countries are under the sway of Russia to one degree or another, and the sanctions the administration imposed on Moscow last week complicates any such discussions, diplomats and military officials said.

Attack planes aboard aircraft carriers and long-range bombers flying from land bases along the Persian Gulf, Indian Ocean and even in the United States could strike insurgent fighters spotted by armed surveillance drones. But the long distances are costly and riskier.

“It’s going to be extremely difficult to do, but it is not impossible,” General McKenzie said under questioning from both Democrats and Republicans on the House Armed Services Committee.

General McKenzie told lawmakers that he was working on a detailed set of alternatives that he would deliver to Defense Secretary Lloyd J. Austin III by month’s end. But he underscored what Mr. Biden’s top intelligence officials have already warned: Without boots on the ground or armed surveillance drones just minutes away, the United States will have far fewer human or electronic eyes and ears focused on extremist threats in Afghanistan.

“The intelligence will decline,” General McKenzie said, “but we’re going to be able to continue to look into Afghanistan.”

American spy agencies do not believe Al Qaeda or other terrorist groups pose an immediate threat to strike the United States from Afghanistan, an assessment that the Biden administration considered pivotal as it weighed continuing the war or pulling out forces this year.

Al Qaeda and the Islamic State’s Afghanistan branch remain very weak inside the country, according to American officials briefed on the intelligence. Islamic State fighters in Afghanistan are focused on making local gains, not mounting international attacks. And the Taliban remains hostile to the group.

How well the United States can monitor any changes in these threats will depend on many factors, General McKenzie and other senior military officials said.

The administration currently plans to maintain an embassy in Kabul — but how many personnel, including intelligence analysts, will remain is under debate. “That will be helpful if we maintain an embassy there,” General McKenzie said.

Flying remotely piloted aircraft like the MQ-9 Reaper from the sprawling Al Udeid Air Base in Qatar requires round trips of up to eight hours, leaving much less time for drones to hover over targets in Afghanistan. As a result, the Pentagon would need many more drones to achieve 24-hour coverage in multiple areas of the country at a time when the Defense Department is looking to transfer surveillance aircraft to missions in Asia.

General McKenzie said that the military would not sacrifice precision in its targeting to minimize the risk of civilian casualties. “We’re going to make sure we have a precise target, and that we’re going to be able to control what happens there,” he said.

The implications of Mr. Biden’s decision continued to sink in on Capitol Hill on Tuesday.

Several top Biden administration officials, including Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, and Zalmay Khalilzad, the longtime lead U.S. diplomat involved in the Afghan peace process, provided classified briefings on Tuesday to several socially distanced groups of lawmakers, and discussed the president’s decision.

General McKenzie made one point clear in his public testimony: No more combat troops. And no more civilian contractors. (There are more than 16,000 in Afghanistan, including about 6,000 Americans.) “We’re going to go to zero in Afghanistan,” he said.

Representative Michael Waltz, a Florida Republican who served in Afghanistan as an Army Green Beret, expressed concern with the lack of ready-to-go alternative bases. “General, do think it would have been optimal to have those agreements before we announced to the world that we’re going to zero in a few months?” Mr. Waltz asked.

General McKenzie said talks with other nations in the region were underway.

Administration officials said they were considering basing drone operations in Central Asian countries, including Uzbekistan, whose Karshi-Khanabad base was a logistics hub for operations in Afghanistan until Uzbek officials ordered the United States to leave in 2005 in protest over a United Nations operation to relocate Uzbek refugees who had fled during an uprising earlier in the year.

Akhror Burkhanov, a spokesman for the Uzbek Embassy in Washington, said he was unaware of any current discussions about possible basing arrangements.

The C.I.A. once flew drone missions out of Shamsi airfield in Baluchistan Province in Pakistan, which has expressed support for the Afghan peace process. But many American and Pakistani analysts say it is highly unlikely Prime Minister Imran Khan would approve any basing arrangements given the fraught history of C.I.A. drone strikes in Pakistan’s tribal regions.

General McKenzie, as well as other military and intelligence officials, noted the advances in surveillance technology since Sept. 11, 2001, as well as myriad ways that the United States had hardened its defenses at home from external terrorist attacks.

“It’s a very different country,” he said, “than it was in the fall of 2001.”