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

At least three people were killed, including a young girl, in an attack the previous day on a children's hospital in Mariupol in southern Ukraine, local officials said on Thursday.

"Three people were killed, including a female child, in yesterday's attack on a children's and maternity hospital in Ukraine's besieged Mariupol, according to updated figures this morning," the city council said on its Telegram channel. Officials had previously given a toll of 17 injured in the attack.

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

"A children's hospital. A maternity hospital. How did they threaten the Russian Federation?" Zelenskyy asked in his nightly video address, switching to Russian to express horror at the strike. "What kind of country is this, the Russian Federation, which is afraid of hospitals, afraid of maternity hospitals, and destroys them?"

He urged the West to impose even tougher sanctions, so Russia "no longer has any possibility to continue this genocide."

Zelenskyy shared video footage showing massive destruction at the recently refurbished hospital in the southern port city, condemning the attack as a "war crime." He 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. The Kremlin said Thursday it would approach the Russian military for details of the strike.

"We will certainly ask our military about this, since we don't have clear information about what happened there. Without fail, the military will provide some kind of information," Kremlin spokesperson Dmitry Peskov told reporters.

Russia's Foreign Minister Sergey Lavrov dismissed concerns about military attacks on civilians, including on a maternity hospital, as "pathetic shrieks" from its enemies. Lavrov met his Ukrainian counterpart Dmytro Kuleba in Turkey on Thursday in the highest-level Russian-Ukrainian talks since the war began last month.

In the Russian government's first public comment on Wednesday's strike on a maternity hospital in the besieged city of Mariupol, Lavrov didn't deny or shy away from responsibility for the attack. He claimed the site had earlier been seized by Ukrainian far-right radical fighters who were using it as a base. Even though there were many images of civilians wounded in the attack and the city council said a child was among the three people killed, Lavrov claimed all the patients and nurses were moved of the hospital before the assault.

Lavrov said Russia was ready for more negotiations but showed no sign of softening Moscow's stance in the dispute.

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 U.N. spokesperson said no health facility "should ever be a target."

U.S. Secretary of State Antony J. Blinken condemned Russia's "unconscionable attacks" in a call with his Ukrainian counterpart, Dmytro Kuleba, the State Department said.

The attack came as women were in labor inside, the regional military administration in Donetsk told Agence France-Presse (AFP). Local authorities hurried to bury the dead from the past two weeks of fighting in a mass grave in the city.

Workers dug a trench some 25 meters (yards) long at one of the city's old cemeteries and made the sign of the cross as they pushed in bodies wrapped in carpets or bags.

The World Health Organization (WHO) said Wednesday that it has confirmed 18 attacks on medical facilities since the Russian invasion began two weeks ago.

Despite often heavy shelling on populated areas, American military officials reported little change on the ground in the last 24 hours, other than Russian progress on the cities of Kharkiv and Mykolaiv, in heavy fighting. The officials spoke on condition of anonymity to assess the military situation.

Zelenskyy said three humanitarian corridors operated on Wednesday, from Sumy in the northeast near the Russian border, from suburbs of Kyiv and from Enerhodar, the southern town where Russian forces took over a large nuclear plant. In all, he said, about 35,000 people got out. More evacuations were planned for Thursday.

People streamed out of Kyiv's suburbs, many headed for the city center, as explosions were heard in the capital and air raid sirens sounded repeatedly. From there, the evacuees planned to board trains bound for western Ukrainian regions, currently not under attack.

Civilians leaving the Kyiv suburb of Irpin were forced to make their way across the slippery wooden planks of a makeshift bridge because the Ukrainians blew up the concrete span leading to Kyiv days ago to slow the Russian advance.

With sporadic gunfire echoing behind them, firefighters dragged an elderly man to safety in a wheelbarrow, a child gripped the hand of a helping soldier, and a woman inched her way along, cradling a fluffy cat inside her winter coat. They trudged past a crashed van with the words "Our Ukraine" written in the dust coating its windows.

In Mariupol, a city of 430,000 people on the Sea of Azov, local authorities hurried to bury the dead from the past two weeks of fighting in a mass grave. City workers dug a trench some 25 meters (82 feet) long at one of the city's old cemeteries and made the sign of the cross as they pushed bodies wrapped in carpets or bags over the edge.

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 toward 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. 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.

Britain's Defense Ministry said fighting continued northwest of Kyiv. Kharkiv, Chernihiv, Sumy and Mariupol were heavily shelled and remained encircled by Russian forces.

Russian forces are placing military equipment on farms and amid residential buildings in the northern city of Chernihiv, Ukraine's military said. In the south, Russians in civilian clothes are advancing on the city of Mykolaiv, a Black Sea shipbuilding center of a half-million people, it said.

The Ukrainian military, meanwhile, is building up defenses in cities in the north, south and east, and forces around Kyiv are "holding the line" against the Russian offensive, authorities said.

U.S. rejects fighter jet plan

Around 2.2 million refugees left Ukraine in what the U.N. 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 U.N.'s atomic watchdog said Wednesday it saw "no critical impact on safety" at Chernobyl, the 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 U.S. 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 U.S. airbase for fear of being drawn into the conflict directly.

Zelenskyy has repeatedly appealed to 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 defenses 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 (IMF) has also approved a $1.4 billion emergency package for Kyiv to provide "critical financial support."

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

British Foreign Secretary Liz Truss Wednesday urged the entire G-7 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.