MARIUPOL, Ukraine (AP) — An attempt to evacuate civilians from the besieged port of Mariupol was thrown into jeopardy by continued fighting Tuesday as conditions grew more desperate inside the city of 430,000, with corpses in the streets and hungry residents breaking into stores for food.

As Ukraine pleaded for more warplanes, Poland said it would give all of its MiG-29 fighter jets to the U.S., apparently agreeing to an arrangement that would allow them to be used by Ukraine’s military. Ukrainian pilots are trained to fly Soviet-era fighter jets.

Ukrainian officials said renewed Russian shelling and other risks endangered the effort to relieve an encircled Mariupol, where the sound of artillery fire was relentless and where thousands crammed into basements. Many were forced to get their water from streams or by melting snow.

Europe’s worst refugee crisis since World War II became even more severe, with U.N. officials reporting that 2 million people have now fled Ukraine.

Moscow’s forces have laid siege to Ukrainian cities and cut off food, water, heat and medicine in an escalating humanitarian disaster. But for days, attempts to create corridors to safely evacuate civilians have stumbled amid continuing fighting and objections to the proposed routes.

Ukrainian President Volodymyr Zelenskyy vowed that his country would fight Russia’s invasion in its cities, fields and riverbanks.

“We will not give up and we will not lose,” he said in a video hookup to a packed House of Commons chamber in the British Parliament, evoking the “never surrender” speech that Winston Churchill gave in the darkest days of World War II.

One evacuation attempt Tuesday did appear at least partially successful: A convoy of buses packed with people fleeing the fighting moved along a snowy road from Sumy, a northeastern city of a quarter-million people, according to video from the Ukrainian communications agency.

The Russian military said 723 people were evacuated from Sumy to the Ukrainian city of Poltava. It identified them as mostly citizens of India, with the rest from China, Jordan and Tunisia. It made no mention of any Ukrainians among those evacuated.

Hours before the convoy reached Sumy, overnight strikes killed 21 people there, including two children, Ukrainian authorities said.

Ukrainian officials also said a safe corridor had been opened from the embattled town of Irpin, outside Kyiv, but it was not clear for how long it remained open and how many people used it

Meanwhile, buses emblazoned with red cross symbols carried water, medicine and food toward Mariupol, scene of some of the worst desperation. Vereshchuk said the vehicles would then ferry civilians out of the city.

But soon after officials announced that buses were on their way, Ukrainian authorities said they had learned of shelling on the escape route.

It was unclear whether the supply convoy made it to Mariupol. And it appeared unlikely that civilians would be able to board the buses to get out.

The deputy mayor of Mariupol told the BBC that Russian forces continued to pound areas where people were trying to gather ahead of being taken out. He said some roads were blocked, while others were mined.

“So we cannot establish a sustainable cease-fire and safety route at the moment,″ Serhiy Orlov said. “So we still have … a city in blockade.’’

U.S. defense officials said Mariupol had been isolated but not yet taken by the Russians, amid stiff resistance from Ukrainian forces.

The capture of Mariupol could allow Moscow to establish a land corridor to Crimea, which Russia seized from Ukraine in 2014. More broadly, the battle appeared to be part of a campaign by the Kremlin to cut off Ukraine’s access to the sea, in what would be a heavy blow to its economy. Mariupol is on the Sea of Azov, which opens onto the Black Sea.

The city is without water, heat, working sewage systems or phone service. Authorities planned to start digging mass graves for all the dead.

With the electricity out, many people are relying on their car radios for information, picking up news from stations broadcast from areas controlled by Russian forces or Russian-backed separatists.

Theft has become widespread for food, clothes, even furniture, with locals referring to the practice as “getting a discount.”

Goma Janna wept as she sat by an oil lamp in a Mariupol basement crowded with people, mostly women and young children. Outgoing artillery fire could be heard in the background.

“Why shouldn’t I cry? I want my home, I want my job. I’m so sad about people and about the city, the children,” she said.

Elsewhere in the city, Ludmila Amelkina walked along an alley strewn with rubble and walls pocked by gunfire, describing the destruction.

“We don’t have electricity, we don’t have anything to eat, we don’t have medicine. We’ve got nothing,” she said, looking skyward.

Nearly two weeks into the fighting, Russian forces have captured a swath of southern and coastal Ukraine. But they have seen their advances stopped in many areas — including around Kyiv, the capital — by nimble Ukrainian fighters targeting Moscow’s armored columns.

Thousands of people are thought to have been killed, both civilians and soldiers, though the actual number remains unknown.

Western countries have rushed weapons to Ukraine and moved to slap Vladimir Putin’s Russia with sanctions.

In a further effort to punish Russia, U.S. President Joe Biden decided to ban Russian oil imports, calling it a “powerful blow” against Russia’s ability to fund the offensive. He warned that Americans will see rising prices, saying: “Defending freedom is going to cost.”

Other financial dominoes fell through the day, with Shell announcing it will stop buying natural gas and oil from Russia, while Adidas and McDonald’s said they are shutting down their Russia operations but will continue paying their employees.

The exact status of the humanitarian corridors that the two sides have been negotiating was not clear. The Russian military said it proposed safe corridors from Kyiv, Chernihiv, Sumy, Kharkiv and Mariupol — two for each city, one leading toward Russia and the other toward the West.

It said that the Ukrainian side accepted only one of those 10 corridors — the one from Sumy to Poltava. Ukrainian officials have rejected the idea of sending civilians to Russia, but there was no immediate word on whether they had turned down those other corridors.

Oleksiy Kuleba, governor of the Kyiv region, said Ukraine was also making arrangements to get people out of the suburb of Irpin.

Late Tuesday, Zelenskyy released a video showing him standing near the presidential offices in Kyiv. Behind him were piles of sandbags, a snow-dusted fir tree and a few cars.

It was the second video in 24 hours showing him near the country’s seat of power, apparently made to dispel any doubts about whether he had fled the city.

“Snow fell. It’s that kind of springtime,” he said in a soft voice. “You see, it’s that kind of wartime, that kind of springtime. Harsh. But we will win.”

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