The United States on Wednesday condemned as "barbaric" the bombing of a children's hospital in Ukraine, where officials said a Russian air strike buried patients in rubble despite a ceasefire deal for people to flee the besieged city of Mariupol.

The attack, which authorities said injured women in labour and left children in the wreckage, is the latest grim incident of the 14-day invasion, the biggest assault on a European state since 1945.

The destruction took place despite a Russian pledge to halt firing so at least some trapped civilians could escape the city, where hundreds of thousands have been sheltering without water or power for more than a week.

Kremlin spokesman Dmitry Peskov, asked by Reuters for comment, said: "Russian forces do not fire on civilian targets." Russia calls its incursion a "special operation" to disarm its neighbour and dislodge leaders it calls "neo-Nazis."

Ukraine's 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.

"It is horrifying to see the type of the barbaric use of military force to go after innocent civilians in a sovereign country," White House press secretary Jen Psaki said.

The U.N. Human Rights body said its monitoring mission was verifying the number of casualties at Mariupol.

"The incident adds to our deep concerns about indiscriminate use of weapons in populated areas and civilians trapped in active hostilities in numerous areas," said spokesperson Liz Throssell.

The Donetsk region's governor said 17 people were wounded in the incident. The city council said the hospital had been hit several times by an air strike, causing "colossal" destruction.

Ukraine accused Russia of breaking the ceasefire around the southern port.

"Indiscriminate shelling continues," Foreign Minister Dmytro Kuleba wrote on Twitter.

Satellite image company Maxar said images from earlier in the day showed extensive damage to homes, apartment buildings, grocery stores and shopping centres in Mariupol.

Russia blamed Ukraine for the failure of the evacuation.

Among more than 2 million total refugees from Ukraine, the United Nations Children's Fund (UNICEF) said on Wednesday that more than 1 million children have fled the country since the invasion started. At least 37 had been killed and 50 injured, it said.

Around 48,000 Ukrainians have been evacuated through humanitarian corridors, Interfax Ukraine news agency said on Wednesday, citing a senior aide to President Volodymyr Zelenskiy.

Local officials said while some had departed from certain locations, Russian forces were preventing buses from evacuating civilians from Bucha, a town near Kyiv.

The International Committee of the Red Cross (ICRC) said houses had been reduced to rubble all across Ukraine. "Hundreds of thousands of people have no food, no water, no heat, no electricity and no medical care."

Thousands continued to flood into neighbouring countries. After hiding in the basement to shelter from Russian bombing, Irina Mihalenka left her home northeast of the Black Sea port of Odessa, she told Reuters in Isaccea, Romania.

"When we were walking, a bridge was blown up. And when we crossed over the wreckage, because there was no other way out, there were corpses of Russian people (soldiers) lying there," she said.

RUSSIA'S ECONOMIC ISOLATION

Pressure on Moscow intensified on Wednesday. Russia has been hit by Western sanctions and the withdrawals of foreign firms since the invasion began on Feb 24. Nestle, cigarette maker Philip Morris and Sony on Wednesday joined the list of multinationals stepping back from the country.

The United States is weighing sanctions on nuclear power supplier Rosatom, a senior Biden administration official said on Wednesday.

The World Bank's chief economist said Moscow was edging close to defaulting on its debt. The Kremlin is taking measures to shore up the economy and planned to respond to a U.S. ban on its oil and energy exports as the rouble dropped to record lows.

There was not much hope for diplomacy as Russia's Foreign Minister Sergei Lavrov arrived in Turkey ahead of talks on Thursday with Kuleba in what will be the first meeting between the pair since the incursion.

Ukraine is seeking a ceasefire, liberation of its territories and to resolve all humanitarian issues, Kuleba said, adding: "Frankly...my expectations of the talks are low."

Moscow demands that Kyiv take a neutral position and drop aspirations of joining the NATO alliance.

The West says Russia is inventing pretexts to justify an unprovoked war. Russian President Vladimir Putin has called Ukraine a U.S. colony with a puppet regime and no tradition of independent statehood.

The White House on Wednesday said Russia's claims about alleged U.S. involvement in biological weapons labs and chemical weapons development in Ukraine were false.

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

A Russian assault force is stalled north of Kyiv and Western countries say the Kremlin has had to adjust its plan to swiftly topple the government.

CHERNOBYL

Ukraine's nuclear power plant operator expressed concern for safety at Chernobyl, mothballed site of the world's worst nuclear disaster in 1986, where it said a power cut caused by fighting meant spent nuclear fuel could not be cooled. Russia's defence ministry blamed Ukraine for the power cut.

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".