KYIV, March 22 - Ukrainian forces fought off continuing Russian efforts to occupy Mariupol and claimed to have retaken a strategic suburb of Kyiv on Tuesday, mounting a defense so dogged that it is stoking fears Russia’s Vladimir Putin will escalate the war to new heights, The Associated Press reported.

“Putin’s back is against the wall,” said U.S. President Joe Biden, who is heading to Europe this week to meet with allies. “And the more his back is against the wall, the greater the severity of the tactics he may employ.”

Biden reiterated accusations that Putin is considering resorting to using chemical or biological weapons, though Pentagon spokesman John Kirby said the U.S. has seen no evidence to suggest that such an escalation is imminent.

Explosions and bursts of gunfire shook Kyiv, and heavy artillery fire could be heard from the northwest, where Russia has sought to encircle and capture several of the capital’s suburban areas.

Early Tuesday, Ukrainian troops drove Russian forces from the Kyiv suburb of Makariv after a fierce battle, Ukraine’s Defense Ministry said. The regained territory allowed Ukrainian forces to retake control of a key highway and block Russian troops from surrounding Kyiv from the northwest.

Still, the Defense Ministry said Russian forces partially took other northwest suburbs, Bucha, Hostomel and Irpin, some of which have been under attack almost since Russia invaded nearly a month ago.

A Western official, speaking on condition of anonymity to discuss military assessments, said Ukrainian resistance has brought much of Russia’s advance to a halt but has not sent Moscow’s forces into retreat.

Western officials say Russian forces are facing serious shortages of food, fuel and cold weather gear, leaving some soldiers suffering from frostbite. Ukrainians have reported hungry soldiers looting stores and homes for food.

The invasion has driven more than 10 million people from their homes, almost a quarter of Ukraine’s population, according to the United Nations.

Thousands of civilians are believed to have died. Estimates of Russian military casualties vary widely, but even conservative figures by Western officials are in the low thousands.

On Monday, Russia’s pro-Kremlin Komsomolskaya Pravda newspaper, citing the Defense Ministry, reported that almost 10,000 Russian soldiers had been killed. The report was quickly removed, and the newspaper blamed hackers. The Kremlin refused to comment. The Western official said the figure is “a reasonable estimate.”

Facing unexpectedly stiff resistance that has left the bulk of Moscow’s ground forces miles from the center of Kyiv, Putin’s troops are increasingly concentrating their air power and artillery on Ukraine’s cities and civilians.

Talks to end the fighting have continued by video. Ukrainian President Volodymyr Zelenskyy has said he would be prepared to consider waiving any bid by Ukraine to join NATO — a key Russian demand — in exchange for a cease-fire, the withdrawal of Russian troops and a guarantee of Ukraine’s security. (ap/ez)