KYIV, Ukraine (AP) — Residents of the bombarded suburbs of Ukraine's capital snaked their way across the slippery wooden planks of a makeshift bridge that provided the only way to escape Russian shelling, amid renewed efforts Wednesday to rescue civilians from besieged cities.

With sporadic gunfire echoing behind them, firefighters dragged an elderly man to safety in a wheelbarrow, a child gripped the hand of a helping soldier, and a woman inched her way along cradling a fluffy cat inside her winter coat. On the far side of the bridge, they all trudged past a crashed van with the words "Our Ukraine" written in the dust coating its windows.

"We have a short window of time at the moment,'' said Yevhen Nyshchuk, a member of Ukraine's territorial defense forces. "Even if there is a cease-fire right now, there is a high risk of shells falling at any moment."

Thousands of people are thought to have been killed, both civilians and soldiers, in two weeks of fighting since President Vladimir Putin's forces invaded Ukraine. The U.N. estimates that more than 2 million people have fled the country, the biggest exodus of refugees in Europe since the end of World War II.

The crisis is likely to get worse as Russian forces step up their bombardment of cities throughout the country in response to stronger than expected resistance from Ukrainian forces. Russian losses have been "far in excess" of what Putin and his generals expected, CIA Director William Burns said Tuesday.

In addition to the humanitarian crisis unfolding in many of Ukraine's cities, concerns for the safety of its nuclear plants amid fighting has raised alarm worldwide.

On Wednesday, the decommissioned Chernobyl nuclear site was knocked off the power grid and forced to switch onto generators. That raised concern about the plant's ability to keep nuclear fuel safely cool, though the U.N. nuclear watchdog said it saw "no critical impact on safety" from the power cut.

The diesel generators at Chernobyl, site of the world's worst nuclear accident in 1986, have fuel for 48 hours, power grid operator Ukrenerho said. The plant was shut down in 2000, but the deserted site still stores spent nuclear fuel from Chernobyl and other nuclear plants around Ukraine.

Authorities announced a new cease-fire on Wednesday to allow civilians to escape from towns around the capital, Kyiv, as well as the southern cities of Mariupol, Enerhodar and Volnovakha, Izyum in the east and Sumy in the northeast. Previous attempts to establish safe evacuation corridors have largely failed due to attacks by Russian forces.

It wasn't immediately clear how successful Wednesday's new effort was. There was no word from the crucial port of Mariupol, where days of shelling have largely cut residents off from the outside world and forced them to scavenge for food and water.

But some people did start streaming out of Kyiv's suburbs along an evacuation route that the Ukrainians said both sides had agreed to, even as explosions could be heard in the capital and air raid sirens sounded repeatedly. Many are headed for the city center, from which they board trains bounded for western Ukrainian regions not under attack.

An intensified push by Russian forces could mean "an ugly next few weeks," Burns, the American official, told a congressional committee, warning that Putin was likely to "grind down the Ukrainian military with no regard for civilian casualties."

Still, Ukraine's general staff of the armed forces said the military is building up defenses in cities in the north, south and east, and forces around Kyiv are "holding the line" against the Russian offensive.

That's even as the general staff said Russian forces are placing military equipment on farms and amid residential buildings in the northern city of Chernihiv. In the south, Russians dressed in civilian clothes are advancing on the city of Mykolaiv, a Black Sea shipbuilding center of a half-million people, it said.

Ukrainian resistance is stiffer than many expected — and Western nations are rushing now to bolster their force. Ukraine's president has pleaded repeatedly for warplanes to counter Russia's significant air power, but Western countries have disagreed over how best to do that amid concerns it could raise the risk of the war expanding beyond Ukraine.

Poland late Tuesday offered to give the U.S. 28 MiG-29 fighter planes for Ukraine's use. U.S. officials said that proposal was "untenable," but they would continue to consult with Poland and other NATO allies.

In addition to material support for Ukraine, Western countries have sought to pressure Russia through a series of punishing sanctions. On Tuesday, President Joe Biden upped the ante further, saying said the U.S. would ban all Russian oil imports, even if it meant rising costs for Americans.

Energy exports have kept a steady stream of cash flowing to Russia despite otherwise severe restrictions that have largely cut its economy off from the world. McDonald's, Starbucks, Coca-Cola, PepsiCo and General Electric all announced that they're temporarily suspending business in the country, furthering that isolation.

The moves have done little to blunt the conflict so far.

A series of air raid alerts Wednesday morning urged residents of the capital to go to bomb shelters amid fears of incoming missiles. Associated Press reporters later heard explosions.

Such alerts are common, though irregular, keeping people on edge. Kyiv has been relatively quiet in recent days, though Russian artillery has pounded the outskirts of the city.

Kyiv regional administration head Oleksiy Kuleba said the crisis for civilians was growing in the capital, with the situation particularly critical in the city's suburbs.

"Russia is artificially creating a humanitarian crisis in the Kyiv region, frustrating the evacuation of people and continuing shelling and bombing small communities," he said.

Amid the bombardments, authorities have tried repeatedly to evacuate civilians, but many attempts have been thwarted by Russian shelling.

One evacuation did appear successful on Tuesday, with Ukrainian authorities saying 5,000 civilians, including 1,700 foreign students, had managed to escape from Sumy, an embattled northeastern city of a quarter-million people.

But in the south, Russian troops have advanced deep along Ukraine's coastline in an effort to establish a land bridge to Crimea, which Moscow seized from Ukraine in 2014. That has left Mariupol encircled by Russian forces.

On Tuesday, an attempt to evacuate civilians and deliver badly needed food, water and medicine failed, with Ukrainian officials saying Russian forces fired on the convoy before it reached the city, which nearly half of the population of 430,000 is hoping to flee.

Corpses lie in the streets, and people break into stores in search of food and melt snow for water. Thousands huddle in basements, sheltering from the Russian shells pounding this strategic port on the Azov Sea.

"Why shouldn't I cry?" resident Goma Janna demanded as she wept by the light of an oil lamp below ground, surrounded by women and children. "I want my home, I want my job. I'm so sad about people and about the city, the children."

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Karmanau reported from Lviv, Ukraine. Associated Press journalists Felipe Dana and Andrew Drake in Kyiv, along with reporters from around the world, contributed to this report.

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Follow the AP's coverage of the Ukraine crisis at https://apnews.com/hub/russia-ukraine