An outgunned but resilient Ukrainian military is adopting a two-prong strategy in the face of a flawed but fierce Russian assault, relying on hit-and-run tactics and the fortification of major cities as President Vladimir Putin's campaign enters a more perilous phase, military experts said.

The odds remain stacked against Ukraine, even as it exceeds Western intelligence assessments that had predicted Kyiv, the capital, would fall within days. Russian forces have begun employing siege tactics, aiming to flatten civilian infrastructure and exact maximum punishment for Ukrainian resistance. While Russia has mostly failed, so far, to seize major cities and effectively supply its soldiers with food and fuel, the Pentagon believes it is probable that Russia will regroup and press its massive advantage in firepower.

John Spencer, a retired Army officer who studies urban warfare for the Madison Policy Forum, said Ukraine's top objective is to make the war as bloody as possible for Russia, as it does not appear Putin will withdraw anytime soon. Giving up secondary cities may become necessary to allow the Ukrainian government to endure in the capital as long as possible, he said.

"Not losing is winning in this case," Spencer said. "The Ukrainian strategy is not to lose."

Russia has bombarded Kyiv mostly with artillery and other long-range weapons, as a 40-mile convoy of tanks and other combat vehicles effectively remains stalled about 15 miles north of the capital and bogged down with logistical setbacks, a senior U.S. defense official told reporters Friday on the condition of anonymity, citing ground rules established by the Pentagon.

In other parts of the country, however, Russia's advances are more significant.

In the northeastern city of Kharkiv, Ukraine's second-largest, Russian forces have reached the outskirts after pounding neighborhoods with missiles, rockets and other weapons for days, the senior U.S. defense official said.

In the south, Putin's forces also have advanced on the strategically important port city of Mariupol, occupied the smaller cities of Berdyansk, Melitopol and Kherson, and appear to have set their sights farther west on Mykolaiv, a city of about 470,000 people. Continued success there would allow the Russians to look even farther west and assault Odessa, a port city of nearly 1 million people, possibly both by land and an amphibious assault by naval forces on ships in the Black Sea.

Ukrainian officials have indicated that after surviving the initial invasion, they intend to launch a counteroffensive against Russian forces. That is likely to favor armed ambushes rather than a major, open assault on the numerically superior Russian military, said Douglas London, a retired senior CIA officer and an analyst at the Middle East Institute.

"They're not going to be able to mount a major counterattack, organize the troops and go, 'Charge!' It's going to be an asymmetrical effort to break up Russian attacks," London said. "The Russians have a lot of static targets because of their own logistical problems, and that's just gold for a harassing special-operations force operating in the rear. That's just what they do."

London predicted that, given Russia's vast military advantage, Putin will press his advantage and Ukrainian forces will eventually have to adjust again.

"I don't see him going gently into that good night," London said of Putin. "Ukraine will eventually lose the cities, or reach a point where they can't resupply them, and they'll have to transition to more of an insurgency or urban underground" operation.

Attacking Russian support lines already has proved to be a significant strategy to reduce Russia's advantages, said Andriy Zagorodnyuk, a former defense minister of Ukraine. Russian vehicles are spread across a wide area on multiple fronts, and they're constantly idling to keep soldiers warm in cold weather, Zagorodnyuk said, making diesel replenishment an enormous challenge.

The Ukrainian military has advised civilian defense volunteers to ignore armored vehicles and instead attack fuel trucks, which are unarmored and often driven by poorly trained Russian soldiers. Cutting off the fuel supply turns tanks and rocket artillery vehicles into road obstacles, said Zagorodnyuk, now chairman of the Center for Defense Strategies, a Ukrainian think tank.

With Russian vehicles holding in place, Ukrainian forces have attacked, leaving smoldering wrecks and giving Russian soldiers a grim choice: stay inside their vehicle where they're vulnerable to a possible missile strike, or try to escape on foot and face the likelihood of being shot or captured. Videos posted on social media and authenticated by The Washington Post show some Russian vehicles were probably abandoned.

Russia's advantage is fighting conventional battles with tanks and mechanized infantry, said Rob Lee, a Russia military expert and a senior fellow with the Foreign Policy Research Institute. The opportunity for Ukraine to undercut that, he said, is to focus attacks with antitank weapons and antiaircraft missiles and target forces in the dark using modern night vision goggles and thermal optics.

Rep. Jason Crow, D-Colo., an Army veteran who fought in Afghanistan and Iraq, visited Ukraine in recent weeks. That its military remains decentralized — one of the deficiencies preventing Ukraine from joining NATO — might be among its best assets in this war, he said in an interview.

"Disparate pockets of resistance, and you're seeing battalion-level units fighting independently, which . . . is maybe a blessing in disguise," Crow said. "It's actually helping them now, because . . . they're not relying on the centralized command-and-control systems.

"They have a unique opportunity now to attrit Russian forces rapidly because they're so messed up."

Spencer, the urban warfare expert, said that inside cities, Ukrainians are increasingly adopting "defense in depth," making it as difficult as possible for invading Russian forces to maneuver easily and survive. Bridges have been blown, tires and barricades have been piled in roads, and citizens have armed themselves with rifles, molotov cocktails and other weapons. He said he is unconvinced that Russia has enough forces to prevent Ukrainians in the capital from receiving additional arms.

History is filled with examples of a relatively small number of fighters digging into a city and its many alleys, rooftops and tunnels, and holding off a large assault for long periods of time, Spencer said. He cited the Battle of Mosul, in which Islamic State militants held out for nine months beginning in fall 2016 as Iraqi forces backed by U.S. air power took back a major city, block by block.