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Ukrainian President Volodymyr Zelenskyy said efforts are under way to evacuate about 18,000 people from Kyiv and embattled towns near the capital.
The efforts are part of broader evacuation attempts via humanitarian corridors in Ukraine, Mr Zelenskyy said. He warned the Russian forces against breaching ceasefire promises.
He appealed again for foreign air support, saying “send us planes”.
While western powers have sent military equipment and beefed up forces on Ukraine’s eastern flank, they have been wary of providing air support and getting drawn into a direct war with Russia.
Mr Zelenskyy also issued an appeal, in Russian, in which he urged Russian soldiers to leave.
“Our resistance for almost two weeks has shown you that we will not surrender, because this is our home. It is our families and children. We will fight until we can win back our land,” he said. “You can still save yourselves if you just go home.”
A humanitarian corridor out of the besieged Ukrainian city of Sumy will continue to function on Wednesday, regional governor Dmytro Zhyvytskyy said.
About 5,000 people took buses out of the north-east city on Tuesday after Moscow and Kyiv agreed on the corridor, he said, and about 1,000 cars were also able to leave, moving towards the city of Poltava in central Ukraine.
An air raid alert was declared on Wednesday morning in and around Kyiv, with residents of the Ukrainian capital urged to go to bomb shelters as quickly as possible.
“Kyiv region – air alert. Threat of a missile attack. Everyone immediately to shelters,” regional administration head Oleksiy Kuleba said on Telegram.
About two weeks into the war, while Russian troops have advanced deep along Ukraine’s coastline, their operation has been brought to a standstill in some areas, including around Kyiv, by fiercer resistance than expected from the Ukrainians.
Russian Foreign Minister Sergey Lavrov was scheduled to fly to Turkey on Wednesday and meet Ukrainian Foreign Minister Dmytro Kuleba and Turkish Foreign Minister Mevlut Cavusoglu on Thursday, Mr Cavusoglu’s office said.
The trilateral meeting will take place on the sidelines of a summit being hosted by Turkey, a Nato member nation.
No further details were announced.
The city of Mariupol, on the Sea of Azov, has been surrounded by Russian soldiers for days and a humanitarian crisis is unfolding in the encircled city of 430,000.
For days, as Moscow’s forces laid siege to Ukrainian cities, attempts to create corridors to safely move out civilians stumbled as fighting continued.
Mariupol is in a “catastrophic situation”, Ukrainian Deputy Prime Minister Iryna Vereshchuk said.
Ukraine will on Wednesday try to evacuate civilians through six humanitarian corridors, including those from Mariupol, Ms Vereshchuk said.
The people of Mariupol have “been effectively taken hostage” by the siege, Natalia Mudrenko, the highest-ranking woman at Ukraine’s UN Mission, told the Security Council.
Authorities in Mariupol planned to start digging mass graves for the dead.
The shelling has destroyed buildings and the city has no water, heat, working sewage systems or phone service.
US rejects Poland's offer
Meanwhile, the Pentagon on Tuesday rejected Poland’s offer to give the US its Mig-29 fighter jets, for use by Ukraine.
Poland’s declaration that it intended to deliver 28 jets to the US Ramstein Air Base in Germany raised the concerning prospect of warplanes departing from a US and Nato base to fly into airspace contested by Russia in the Ukraine conflict, Pentagon spokesman John Kirby said.
“We will continue to consult with Poland and our other Nato allies about this issue and the difficult logistical challenges it presents, but we do not believe Poland’s proposal is a tenable one,” Mr Kirby said.
The Polish foreign ministry announced the plan in a statement, saying the jets would be delivered to Ramstein, free of charge.
“At the same time, Poland requests the United States to provide us with used aircraft with corresponding operational capabilities,” it said.
The Polish government also appealed to other owners of Mig-29 jets to follow suit.
The proposed gift of more warplanes would be a morale booster for Ukrainians reeling under Russia's assault.
But it also raises the risk of the war expanding beyond Ukraine.
Russia has said that supporting Ukraine’s air force would be tantamount to joining the war, and could spur retaliation.
White House officials were blindsided by the Polish announcement on the Migs.
US Undersecretary of State Victoria Nuland told senators at a hearing on the Ukraine crisis on Tuesday that she learnt of Poland’s plans only while driving to the hearing.
“To my knowledge, it wasn’t pre-consulted with us,” Ms Nuland said.
Ukraine has been pleading for more warplanes as it resists more powerful Russian forces.
Washington has been looking at a proposal under which Poland will supply Ukraine with the Mig-29s and in turn receive American F-16s to make up for their loss.
Ukrainian pilots are trained to fly the Soviet-era fighter jets.