Russian President Vladimir Putin's military forces seem poised to adopt siege tactics that could result in enormous civilian deaths and infrastructure damage after their initial assault has failed to topple the Ukrainian government, officials and experts say.

Putin and his military commanders have been caught off guard by the ferocious resistance of Ukrainian troops and ordinary citizens. His main assault force for the capital of Kyiv remains bogged down about 15 miles from the city, harassed by Ukrainian attacks and hobbled by a lack of fuel for combat vehicles and food for the Russian soldiers, a senior Defense Department official said Wednesday, speaking on condition of anonymity to describe intelligence findings.

Putin went to war with a "deeply flawed" plan for a swift operation to decapitate the Ukrainian government and install a puppet regime, said Sen. Richard Blumenthal, D-Conn., a member of the Armed Services Committee. The strategy rested on the assumption that Ukrainian soldiers and citizens would acquiesce without a fight.

"He has gravely miscalculated the courage and resilience of the Ukrainian people," said Blumenthal, who visited Ukraine shortly before the Russian invasion and has been briefed on the war by the Pentagon. "These people were never going to welcome Putin as a liberator, nor would they accept Russian rule modeled on the Soviet example. They’ve gotten used to freedom, and they love it."

Russians likely to use 'classic siege' tactics

The Pentagon believes Russian forces will learn from and adapt to their initial failures, the official said. Russian commanders seem intent on encircling key cities. They're expected to show "increasing aggressiveness in the iron they’re lobbing" into city centers with artillery and rocket fire, the official said.

Russian forces, using increasing barrages of artillery fire, will seek surrender, the official said. The official called it "classic siege behavior," the tactic of surrounding a city, sealing in its residents and battering and starving it into submission.

The Ukrainian cities of Kyiv, Kharkiv and Mariupol have come under heavy bombardment from airstrikes and artillery in the last 24 hours, according to British intelligence estimates.

Syria, Chechnya fighting could serve as blueprints

Putin used the same siege tactics in Syria and Chechnya, Blumenthal said. Russia, for example, bombed the Syrian city of Aleppo for weeks in 2016, likely killing hundreds of civilians, according to Human Rights Watch.

"He has no compunction about loss of life to achieve his really warped, cruel goals," Blumenthal said. "In this case, his goal is to rebuild the empire that was the Soviet Union. A siege is likely and may last as long as several weeks. He’ll try to wear down resistance in Kyiv and Kharkiv and other major cities, hitting civilian centers with massive explosives, airstrikes and missiles."

But even if Putin's siege tactics allow the Russians to enter Kyiv, he doesn't have the troops needed to secure it, said retired Gen. Peter Chiarelli, the former Army vice chief of staff. Putin has deployed more than 150,000 Russian troops to Ukraine's borders, but not all of them are in the country.

A veteran of the Iraq insurgency, Chiarelli estimated as many as 15% of the population in Baghdad wanted to kill U.S. troops. Kyiv will be far different for Russian troops, who will face near-universal resistance.

"You're going to move into a city of 3 million people and not one of them is going to come to your aid," he said. "What a lot of people aren't understanding is how combat in urban areas just sucks up manpower."

The longer the war grinds on, the more frustrated Putin will become, Chiarelli said.

"It's going to get worse," he said. "Quite frankly, that's what scares me. We are all rooting for the Ukrainians to continue to do what they've been able to do so far. But at the same time, a desperate Putin really scares the hell out of me."

His use of tactical nuclear weapons can't be ruled out, Chiarelli said.

"If you look at Russian doctrine, that's something that they seem more willing to use than any other country in the world," he said. "It's a very, very concerning period in the world's history."

Is Vladimir Putin committing war crimes?:

Blumenthal agreed that Putin's unpredictability is dangerous.

"Nobody’s inside his head," he said. "Nobody knows what he is going to do, and whether the impact on his people, namely of sanctions on his economy and casualties, will have an impact."

For now, the United States and Western allies need to continue tightening sanctions and supplying Ukrainian forces, Blumenthal said.

continue to arrive despite the fighting, the Pentagon official said.

"They need more anti-tank and anti-aircraft missiles," Blumenthal said. "More small arms. More of everything."

USA Today