Ukrainian forces battled 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, Associated Press reported.

“Putin’s back is against the wall,” said US 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 US 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. Ukrainian President Volodymyr Zelenskyy accused Russian forces of not only blocking a humanitarian convoy trying to take desperately needed aid to Mariupol but seizing what another Ukrainian official said were 15 of the bus drivers and rescue workers on the aid mission, along with their vehicles.

Zelenskyy said the Russians had agreed to the route ahead of time, according to the Associated Press.

“We are trying to organize stable humanitarian corridors for Mariupol residents, but almost all of our attempts, unfortunately, are foiled by the Russian occupiers, by shelling or deliberate terror,” he said in his nightly video address to the nation.

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 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.

A video posted by Ukrainian police showed them surveying damage in Makariv, including to the town’s police station, which an officer says took a direct hit to its roof. The police drove by destroyed residential buildings and along a road pocked by shelling. The town appeared all but deserted, Associated Press reported.

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.

“We have seen indications that the Ukrainians are going a bit more on the offensive now,” Kirby told reporters separately in Washington. He said that was particularly true in southern Ukraine, including near Kherson, where “they have tried to regain territory.”

Asked on CNN what Russian President Vladimir Putin had achieved in Ukraine, Kremlin spokesman Dmitry Peskov said: “Well, first of all, not yet. He hasn’t achieved yet.” But he insisted that the military operation was going “strictly in accordance with the plans and purposes that were established beforehand.”

Putin’s aims remain to “get rid of the military potential of Ukraine” and to “ensure that Ukraine changes from an anti-Russian center to a neutral country,” Peskov said.

Russia’s far stronger, bigger military has many Western military experts warning against overconfidence in Ukraine’s long-term odds. Russia’s practice in past wars in Chechnya and Syria was to grind down resistance with strikes that flattened cities, killed countless civilians and sent millions fleeing, according to the Associated Press.

But Russian forces appeared unprepared and have often performed badly against Ukrainian resistance. The U.S. estimates Russia has lost a bit more than 10 percent of the overall combat capability it had at the start of the fight, including troops and tanks and other materiel.