WASHINGTON — Ukraine’s soldiers have blown up bridges to halt advancing Russian ground troops. Its pilots and air defenses have prevented Russian fighter jets from conquering the skies. And a band of savvy Ukrainian cyberwarriors are so far beating Moscow in an information war, inspiring support at home and abroad.

To the surprise of many military analysts, Ukrainian troops are mounting a stiffer-than-expected resistance to Russian forces up and down battle lines across a country the size of Texas, fighting with a resourcefulness and creativity that U.S. analysts said could trip up Russian troops for weeks or months to come.

The Ukrainians are also exploiting a bungled beginning to Russia’s all-out assault. Armed with shoulder-fired anti-tank weapons, they have attacked a mileslong Russian armored convoy bearing down on Kyiv, the capital, helping stall an advance plagued by fuel and food shortages, and stretching a march that was expected to take a handful of days into possibly weeks.

To be sure, Russia’s invasion is only a week old. The strategic southern city of Kherson fell on Wednesday; the Kremlin’s army has intensified its bombardment of Kyiv and other cities; and, despite a flow of fresh arms pouring in from the West, Ukrainian leaders say they desperately need more weapons to destroy Russian tanks and down Russian warplanes.

But the Ukrainian military is conducting a hugely effective and mobile defense, using their knowledge of their home turf to stymie Russian forces on multiple fronts, Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, said on Thursday.

General Milley said some of the tactics employed by Ukrainian troops included using mobile weapons systems to bedevil the Russians wherever they could. Ukraine’s forces, he told reporters traveling with him in Europe, are “fighting with extraordinary skill and courage against Russian forces.”

U.S. officials have been impressed with the fighting prowess of the Ukrainians, but their assessment that Russia has the superior military has not changed.

Ukraine has succeeded in slowing the Russian advance, but has not been able to stop it, nor is the resistance strong enough to shift Mr. Putin’s war aims. But over the long term, U.S. officials said, it will be difficult for Ukraine to continue to frustrate the Russian advance.

In the meantime, though, Ukrainians are turning into a nation at arms. “In combat, it’s always different than what you thought it’d be, and the side that learns faster and adapts faster will win out,” said Frederick B. Hodges, the former top U.S. Army commander in Europe who is now with the Center for European Policy Analysis. “So far, Ukraine is learning and adapting faster.”

Ukraine has one of Europe’s largest militaries, with 170,000 active-duty troops, 100,000 reservists and territorial defense forces that include at least 100,000 veterans. Thousands of civilians are also now enlisting.

The Ukrainian army has been training for further Russian encroachment ever since Russia seized Ukraine’s Crimean Peninsula in 2014 and began supporting separatists in the Donbas region, in eastern Ukraine. Many of Ukraine’s veterans fought in those battles, so there is a subset of the population that is trained and knows how to fight Russians.

U.S. Special Operations Forces have also trained Ukrainian military forces. Leaders in Kyiv then assigned those soldiers to conventional units, allowing them in turn to train a larger portion of the army. American analysts say that training has made a difference on the battlefield.

The United States has provided more than $3 billion in weapons, equipment and other supplies to Ukraine’s armed forces since 2014. In those eight years, U.S. military advisers, including Army Green Berets and National Guard troops, have trained more than 27,000 Ukrainian soldiers at the Yavoriv Combat Training Center near Lviv in western Ukraine.

In Brussels on Thursday, NATO Secretary General Jens Stoltenberg said the Ukrainian military was “performing better and putting up more resistance than most experts expected, and surely more than Russia expected.”

“They’re there to defend their own land,” Mr. Stoltenberg told reporters traveling with General Milley.

Indeed, Michael R. Carpenter, the U.S. representative to the Organization for Security and Cooperation in Europe, hailed a Ukrainian marine, Vitaliy Skakun, in remarks in Vienna on Thursday. The marine had blown himself up on a bridge in the southern Kherson region to prevent a line of Russian tanks from crossing, the Ukrainian military said.

From the invasion’s opening hours, Ukraine’s underdog military has sought to flip the script on the more than 150,000 Russian forces massed on its borders. For instance, Ukrainian troops repelled an attack by Russian airborne and special forces on a key airfield north of Kyiv last Thursday in the initial hours of the war, thwarting a Russian attempt to open a major air bridge on the outskirts of the capital.

“In city defense and skirmishing on the outskirts of cities, Ukrainian forces are doing quite well,” said Michael Kofman, director of Russia studies at CNA, a defense research institute. “The shambolic nature of the Russian war effort undoubtedly helps.”

As Russians approached Kyiv and Kharkiv, the Ukrainians were able to shift their forces to critical locations faster than the invading forces. Not only have the Ukrainians moved more nimbly, they also made good choices about where to concentrate firepower.

“The art of mechanized maneuver warfare is being able to concentrate overwhelming combat power at decisive sections of the front, places of your choosing,” said Frederick W. Kagan, a military strategist who has advised the U.S. command in both Iraq and Afghanistan. “The Russians, astonishingly, failed to do that. But the Ukrainians have taken advantage of their ability to move reinforcements rapidly and counterattack.”

Thomas Bullock, an open-source analyst from Janes, the defense intelligence firm, said Russian forces have made tactical errors that the Ukrainians have been able to capitalize on.

“It looks like the Ukrainians have been most successful when ambushing Russian troops,” Mr. Bullock said. The Russians “have stuck to main roads so that they can move quickly and not risk getting bogged down in mud. But they are advancing on winding roads and their flanks and supply routes are overly exposed to Ukrainian attacks. And that is where they have had their most success.”

In Kyiv, the Ukrainian counterattack has pushed the Russian troops west and forced them to call in reinforcements as they try to encircle the city, said Mr. Kagan, an expert on the Russian military who leads the Critical Threats Project at the American Enterprise Institute.

While it is often easier to defend than attack, especially in a complex multifront invasion, the Ukrainians have taken advantage of the Russian decision to use too small a force, sometimes only two battalions at a time, to take key points.

“They have been much more evenly matched at the tactical level than they should have been, had the Russians conducted the operations well,” Mr. Kagan said. “The Ukrainians have just been much smarter about this than the Russians.”

The Ukrainians have been far more successful in the north, defending Kyiv and Kharkiv, the country’s second-largest city, than they have been in the south, where better trained Russian forces in Crimea have had more success.

“In the south, on the Crimean front, when the Ukrainians are engaged in mechanized combat they are losing,” Mr. Bullock said.

However, the public might not be aware of that because of a bias in the information that is available to them. The Ukrainian government has publicized its victories and Russian attacks that killed civilians, but not battlefield losses of its mechanized units.

“The Ukrainians have been more successful than we expected them to be, but there is an availability bias in the open source,” Mr. Bullock said. “We see Ukraine’s success and prowess. We are not seeing as much about Ukraine’s losses.”

In addition, because Russian officials are keen not to present the operation as a war, they have not put out information about the engagements their forces have won.

The result, in these early days of the invasion, is that the Ukrainians are beating the Russians at their own information campaign.

“The Ukrainians are winning the information war. That is quite impressive considering the prowess of the Russians,” Mr. Bullock said. “The Ukrainians have flooded the information space with their narrative and their side of the story to mobilize support domestically and internationally.”

Still, U.S. government officials believe that Mr. Putin is likely to redouble his assault. But some analysts say that growing Russian casualties, increasing economic disruptions in Russia resulting from sanctions, and the likelihood of an enduring Ukrainian insurgency could upend that strategy.

Eric Schmitt and Julian E. Barnes reported from Washington, and Helene Cooper from Brussels.