International leaders and Ukraine accused Russia of a "barbaric" attack on a children's hospital in the besieged city of Mariupol, as civilians continued to bear the brunt of the conflict two weeks into Moscow's invasion.

Ukrainian President Volodymyr Zelensky said 35,000 civilians had managed to flee cities under Russian attack on Wednesday, but there was little relief in Mariupol where the mayor said relentless bombardment had killed over 1,200 civilians in the nine-day siege.

Zelensky shared video footage showing massive destruction at the recently refurbished hospital in the southern port city, condemning the attack as a "war crime."

A local official said the attack wounded at least 17 staff, though no deaths were immediately reported. Zelensky said the "direct strike by Russian troops" had left children under the wreckage.

Russia's foreign ministry did not deny the attack but accused Ukrainian "nationalist battalions" of using the hospital to set up firing positions after moving out staff and patients.

Video shared from the site by rescue workers showed a scene of complete devastation, with the wounded being evacuated, some on stretchers, past charred and burning carcasses of cars and a massive crater by the building.

Inside, debris, shattered glass and splintered wood littered corridors, administrative offices and bedrooms, with mattresses thrown from their frames.

The White House slammed the "barbaric" use of force against civilians, while British Prime Minister Boris Johnson called the attack "depraved".

A UN spokesman said no health facility "should ever be a target".

The attack came as women were in labour inside, the regional military administration in Donetsk told AFP. 

- Talks in Turkey -

It took place on the eve of the highest-level talks to date between the two nations.

Russian Foreign Minister Sergei Lavrov landed in Turkey for the face-to-face talks set for Thursday with his Ukrainian counterpart Dmytro Kuleba -- who warned in a Facebook video his expectations were "limited".

So far, the parties have been engaged in lower-level talks in Belarus, largely over humanitarian issues and involving Ukrainian officials but no Russian ministers.

Those discussions have produced several attempts to bring civilians out of cities under attack, many of which have failed after so-called humanitarian corridors came under attack.

But on Wednesday, at least 35,000 civilians were able to leave the cities of Sumy, Enerhodar and areas around Kyiv, Zelensky said.

He said he hoped the evacuations would continue on Thursday with three more routes set to open out of  Mariupol, Volnovakha in the southeast and Izium in eastern Ukraine.

Previous attempts to allow civilians to leave Mariupol in particular have collapsed, with aid groups warning of a catastrophic situation in the city where basic services have halted.

And Moscow's forces have continued making rapid advances towards the capital, approaching Brovary, a large eastern suburb, AFP journalists saw.

Fighting has intensified in the area, with Ukrainian forces trying to repel the Russian tanks, residents and volunteer Ukrainian forces told AFP. 

"They shoot to scare people and force them to stay at home, steal what they can to get supplies and settle among the inhabitants, so that the Ukrainian forces do not bomb them," said Volodymyr, a 41-year-old resident of Velyka Dymerka, 15 kilometres (nine miles) from Brovary.

Overnight, the Ukrainian General Staff said Russian forces were continuing their "offensive operation" to encircle Kyiv, while pressing attacks on a string of other cities across the country. 

- US rejects fighter jet plan -

Russia's war has sent around 2.2 million refugees across Ukraine's borders in what the United Nations has called Europe's fastest-growing refugee crisis since World War II.

The conflict has raised fears of a nuclear accident in a country with major nuclear plants and the site of the Chernobyl disaster.

The UN's atomic watchdog said Wednesday it saw "no critical impact on safety" at Chernobyl, location of the world's worst nuclear disaster in 1986, despite a loss of power there.

But it warned it was not receiving updates from either Chernobyl or Zaporizhzhia, Europe's largest nuclear plant, which is also now under Russian control.

The United States meanwhile rejected Russian claims that it was involved in bioweapons research in Ukraine, and warned Russia could be preparing to use chemical or biological weapons in the war.

Washington has strongly backed Ukraine, leading the push for tough international sanctions and sending weapons and other aid/

But it has ruled out enforcing a no-fly zone and rejected a Polish plan to transfer fighter jets via a US air base for fear of being drawn into the conflict directly.

Zelensky has appealed repeatedly for Western powers to find a way to provide it with Poland's Soviet-era MiG-29 fighter jets, which Ukrainian pilots already know how to fly.

Washington has however beefed up defences in Poland, where it said Wednesday it was sending two new surface-to-air missile batteries.

And Britain said it was preparing to send more portable missile systems to help Ukraine, in addition to more than 3,000 anti-tank weapons sent so far, while Canada pledged an additional $50 million worth of military equipment.

- Calls for oil ban -

The International Monetary Fund has also approved a $1.4-billion emergency package for Kyiv to provide "critical financial support."

In tandem with military assistance to Kiev, Western allies have sought to squeeze Moscow with unprecedented sanctions -- including a US ban announced Tuesday on the oil imports that help bankroll the conflict.

British Foreign Secretary Liz Truss Wednesday urged the entire G7 to ban Russian oil imports, saying the world's top economies should "go further and faster" in punishing Moscow for invading Ukraine.

But political leaders are wary of the impact, with French Economy Minister Bruno Le Maire warning the current spike in energy prices could produce effects comparable to the 1973 oil shock.

The European Union agreed in the meantime to add more Russian oligarchs to a sanctions blacklist.

35,000 civilians evacuated from cities in one day

At least 35,000 civilians were evacuated from besieged Ukrainian cities on Wednesday, President Volodymyr Zelensky said.

In a video address late Wednesday, the Ukrainian leader said three humanitarian corridors had allowed residents to leave the cities of Sumy, Enerhodar and areas around Kyiv.

He said he hoped the evacuations would continue on Thursday with three more routes set to open out of the cities of Mariupol, Volnovakha in the southeast and Izium in eastern Ukraine.

The evacuations came after Moscow and Kyiv agreed on Wednesday to open more corridors, offering a glimmer of hope for terrified civilians trapped in bombarded cities.

More than 5,000 people were evacuated a day earlier from Sumy, a city of 250,000 that lies close to the Russian border and has been the scene of heavy fighting. 

But attempted evacuations from the port town of Mariupol, which has been besieged by Russia for days, have failed on several occasions, with both Kyiv and Moscow blaming each other.

On Wednesday, a Russian strike destroyed a children's hospital in the city, triggering renewed global outrage two weeks into Moscow's invasion of its ex-Soviet neighbour.

Mariupol's mayor said more than 1,200 civilians have been killed in the siege, which has lasted more than a week.

The UN refugee agency UNHCR has estimated the total number of refugees at 2.1 to 2.2 million.