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.

“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.

The warnings came as attacks continued in and around Kyiv and Mariupol, and people escaped the battered and besieged port city.

The hands of one exhausted Mariupol survivor were shaking as she arrived by train in the western city of Lviv.

“There’s no connection with the world. We couldn’t ask for help,” said Julia Krytska, who was helped by volunteers to make it out with her husband and son. “People don’t even have water there.”

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 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.

U.N. Secretary-General Antonio Guterres said he saw progress in the talks.

“From my outreach with various actors, elements of diplomatic progress are coming into view on several key issues,” and the gains are enough to end hostilities now, he said. He gave no details.

The Western official, though, said that there were no signs Moscow was ready to compromise.

In the last update from Mariupol officials, they said March 15 that at least 2,300 people had died in the siege. But there are fears the toll could be much higher. Airstrikes over the past week devastated a theater and an art school where many civilians were taking shelter.

Thousands have managed to flee Mariupol, where the bombardment has cut off electricity, water and food supplies and severed communication with the outside world. The city council said Tuesday that more than 1,100 people who had escaped the siege were in a convoy of buses heading to a city northwest of Mariupol.

But the Red Cross said a humanitarian aid convoy trying to reach the city with desperately needed supplies still had not been able to enter.

Perched on the Sea of Azov, Mariupol is a crucial port for Ukraine and lies along a stretch of territory between Russia and Crimea. The siege has cut the city off from the sea and allowed Russia to establish a land corridor to Crimea.

But it’s not clear how much of the city Russia holds, with fleeing residents saying fighting continues street by street.

A senior U.S. defense official, speaking condition of anonymity to give the Pentagon’s assessment, said Russian ships in the Sea of Azov were shelling Mariupol. The official said there were about seven Russian ships in that area, including a minesweeper and a couple of landing vessels.

Ukraine’s Defense Ministry said that troops defending the city had destroyed a Russian patrol boat and electronic warfare complex. Britain’s Defense Ministry said Ukrainian forces “continue to repulse Russian attempts to occupy” Mariupol.

Those who have made it out of Mariupol told of a devastated city.

“They bombed us for the past 20 days,” said 39-year-old Viktoria Totsen, who fled into Poland. “During the last five days the planes were flying over us every five seconds and dropped bombs everywhere — on residential buildings, kindergartens, art schools, everywhere.”

Beyond the terrible human toll, the war has shaken the post-Cold War global security consensus, imperiled the world supply of key crops, and raised worries it could set off a nuclear accident.

Wildfires broke out near the decommissioned Chernobyl nuclear power plant, but Ukraine’s natural resources minister said the flames had been extinguished and radiation was within normal levels. Chernobyl in 1986 was the scene of the world’s worst nuclear disaster.

As part of a series of addresses to foreign legislatures, Zelenskyy urged Italian lawmakers to strengthen sanctions against Moscow, noting many wealthy Russians have homes in the country.

“Don’t be a resort for murderers,” he said from Kyiv.

(AP)