Ukraine accused Russia on Wednesday of bombing a children's hospital in the besieged port of Mariupol during an agreed ceasefire to enable civilians trapped in the city to escape.

Russia had said it would hold fire to let thousands of civilians flee Mariupol and other besieged cities on Wednesday. But the city council said the hospital had been hit several times by an air strike.

"The destruction is colossal," it said in an online post.

President Volodymyr Zelenskiy called it an "atrocity".

"Direct strike of Russian troops at the maternity hospital. People, children are under the wreckage," he said on Twitter.

Kremlin spokesman Dmitry Peskov, asked by Reuters for comment on the reported bombing, said: "Russian forces do not fire on civilian targets."

The Ukrainian Foreign Ministry posted video footage of what it said was the hospital showing holes where windows should have been in a three-storey building. Huge piles of smouldering rubble littered the scene.

The Donetsk region's governor said 17 people were wounded, including women in labour. The reports could not immediately be verified.

Earlier Ukraine's Foreign Minister Dmytro Kuleba said Russia had broken the ceasefire around the southern port, which lies between Russian-backed separatist areas of eastern Ukraine and Crimea, annexed by Moscow from Ukraine in 2014.

"Russia continues holding hostage over 400,000 people in Mariupol, blocks humanitarian aid and evacuation. Indiscriminate shelling continues," he wrote on Twitter. "Almost 3,000 newborn babies lack medicine and food."

Ukraine said at least 1,170 civilians had been killed in Mariupol since the start of the invasion, and 47 were buried in a mass grave on Wednesday. It was not possible to verify the figures.

Russia's defence ministry blamed Ukraine for the failure of the evacuation and said that the situation faced by civilians in Mariupol had reached a "catastrophic scale".

A senior U.S. defence official said there were indications Russia's military was using so-called "dumb" bombs that are not precision-guided and that Washington had observed "increasing damage to civilian infrastructure and civilian casualties".

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 Bucha town outside Kyiv, local authorities said. Talks continued to allow the convoy to leave, they said.

"In just two weeks, homes have been reduced to rubble," the International Committee of the Red Cross (ICRC) said of the situation in Ukraine.

"Families are huddled underground for hours on end to seek refuge from fighting. Hundreds of thousands of people have no food, no water, no heat, no electricity and no medical care."

More than 2 million people have fled Ukraine since President Vladimir Putin launched the land, sea and air invasion on Feb 24. Moscow calls its action a "special military operation" to disarm its neighbour and dislodge leaders it calls "neo-Nazis."

Russian forces hold territory stretching along Ukraine's northeast border, the east and the southeast. Fighting has taken place in the outskirts of the capital Kyiv, while Ukraine's second city Kharkiv is under bombardment.

The United Nations human rights office in Geneva, just prior to the reports of the hospital attack, said it had verified 516 civilian deaths and 908 people wounded since the conflict began.

CHERNOBYL

Kyiv and its Western allies say Russia is inventing pretexts to justify an unprovoked war against a democratic country of 44 million people. Moscow has accused Ukraine of having tried to develop biological or nuclear weapons.

On Wednesday, the Kremlin said Washington must explain "Ukrainian biological weapons labs", a suggestion Washington has already dismissed as "absurd propaganda".

Ukraine's nuclear power plant operator said it was concerned for safety at Chernobyl, 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.

Foreign minister Kuleba said reserve diesel generators had a 48-hour capacity. "After that, cooling systems of the storage facility for spent nuclear fuel will stop, making radiation leaks imminent," he said.

The International Atomic Energy Agency said the heat generated by the spent fuel and the volume of cooling water were such that it was "sufficient for effective heat removal without need for electrical supply".

'RETALIATORY MEASURES'

The war has swiftly cast Russia into economic isolation as well as drawing almost universal international condemnation.

The United States on Tuesday banned imports of Russian oil, while Western companies are rapidly pulling out from the Russian market. The ruling United Russia party said it proposed seizing the assets of foreign companies that leave.

Both Ukraine and Russia are huge exporters of food and metals. Together they account for nearly a third of the global grain trade. Prices of food staples have soared worldwide, punishing far-flung countries in the Middle East, Africa and Asia.

Ukraine said on Wednesday it was halting key agricultural exports for the rest of the year. Russia too said it needed to maintain domestic supplies of grain.

In the latest sign of what was rapidly becoming a global food crisis, Indonesia said it would curb sales of palm oil after global prices surged.

SURPRISE RESISTANCE

Western countries believe Moscow had aimed to quickly topple the Kyiv government in a lightning strike and is being forced to adjust after underestimating Ukrainian resistance. Russia has taken substantial territory in the south but has yet to capture any big cities in northern or eastern Ukraine, with an assault force stalled on a highway north of Kyiv.

Russia is desperate for some kind of victory in cities like Mariupol and Kyiv, before it negotiates, Vadym Denysenko, an adviser to Ukraine's interior minister, said on Wednesday.

"Therefore, our task is to withstand for the next 7-10 days," he said. (Reuters)

•PHOTO: Debris is seen on site of the destroyed Mariupol children's hospital as Russia's invasion of Ukraine continues, in Mariupol, Ukraine, March 9, 2022 in this still image from a handout video obtained by Reuters. Ukraine Military/Handout via REUTERS

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