Ukraine-Russia War News and Analysis: Live Updates

Ukrainian forces have used an array of battlefield tactics to slow the Russian advance, including blowing up bridges, carrying out small-arms attacks and launching surface-to-air missiles to knock down Russian attack planes and helicopters.

“They’re using everything in their arsenal,” the Pentagon official said of Ukraine’s military.

Russian armored columns are running out of fuel and spare parts faster than Moscow expected, the official said, but it is unclear whether commanders drew up an effective combat plan that they have poorly executed, or whether they did not have a good plan to start with. Russia’s all-out assault is only in its fifth day, and the Pentagon official cautioned that commanders would most likely learn from their failures and adapt.

American officials also fear that Russia, frustrated over its early setbacks, may escalate missile and aerial bombing of cities, causing major civilian casualties.

As fighting intensifies throughout the country, Moscow’s main focus remains on Kyiv, where U.S. officials say the Russian goal is to decapitate the current government and install pro-Moscow leaders. Russian forces have begun adopting siege tactics around Chernihiv, a historic city northeast of Kyiv, the Pentagon said over the weekend, and officials fear that Russia will use a similar medieval strategy against Kyiv.

U.S. military officials had not seen any noticeable change in Russia’s nuclear forces as of Monday, a day after President Vladimir V. Putin ordered those forces on higher alert, the Pentagon official said.

“We’ve not seen anything specific as a result of the direction he gave, at least not yet,” the official said.

A European official said on Monday that Russia was “struggling to convert ambition into solid gains,” pointing to the fact that no major cities have fallen.

The strength of the Ukrainian military’s resistance, the official said, was a “nasty surprise” for Mr. Putin and the Russian military.

Because some Ukrainian air defense remains active, the Russian forces do not have dominance of the skies. The arrival in Ukraine of new Stinger missiles, the official said, could mean that Russia will have to conduct more of its air operations at night.

Under Russia’s original plan, some of its forces in Crimea and on the border of Donbas in eastern Ukraine would have made rapid progress toward Kyiv by this point in the invasion, the official said.

While the Russian forces in the south have been more effective than those coming from Belarus, the Ukrainians have slowed them down and most of those forces have not made it to Kyiv.