Ukraine accused Russia on Wednesday of breaking a ceasefire to prevent the evacuation of hundreds of thousands of civilians trapped in the besieged port of Mariupol, where the Red Cross has described conditions as "apocalyptic".
Russia said it would hold fire to let civilians flee besieged cities, but efforts to evacuate Mariupol appeared to have failed again, as have several previous attempts since Saturday, reports Reuters.
"Russia continues holding hostage over 400,000 people in Mariupol, blocks humanitarian aid and evacuation. Indiscriminate shelling continues," Ukraine's Foreign Minister Dmytro Kuleba wrote on Twitter. "Almost 3,000 newborn babies lack medicine and food."
Local officials in other cities said some civilians had left on Wednesday through safe corridors, including out of Sumy in eastern Ukraine and Enerhodar in the south.
However, Russian forces were preventing a convoy of 50 buses from evacuating civilians from the town of Bucha outside Kyiv, local authorities said in an online post, adding that talks continued to allow the convoy to leave.
Both sides have accused each other of violating ceasefires that would allow evacuating Mariupol, which Russian forces have kept under siege for more than a week.
On Tuesday, the Red Cross called conditions inside the city "apocalyptic", with residents sheltering underground from relentless bombardment, with no access to food, water, power or heat.
More than 2 million people have fled Ukraine since President Vladimir Putin launched the invasion nearly two weeks ago. Moscow calls its action a "special military operation" to disarm its neighbour and dislodge leaders it calls "neo-Nazis."
Kyiv and its Western allies dismiss that as a baseless pretext for an unprovoked war against a democratic country of 44 million people.
In recent days, Russia has also accused Ukraine of having tried to develop biological or nuclear weapons. On Wednesday, the Kremlin said Washington must explain "Ukrainian biological weapons labs". Washington has already dismissed that claim as "absurd propaganda" and accused Russia of seeking retroactive pretexts for the war.
Ukraine's nuclear power plant operator said it was concerned for safety at Chernobyl, the mothballed site of the world's worst nuclear disaster, where it said a power cut caused by fighting meant spent nuclear fuel could not be cooled.
"Reserve diesel generators have a 48-hour capacity," foreign minister Kuleba tweeted. "After that, cooling systems of the storage facility for spent nuclear fuel will stop, making radiation leaks imminent."
The International Atomic Energy Agency said in a statement the heat generated by the spent fuel and the volume of cooling water were such that it was "sufficient for effective heat removal without the need for electrical supply."
