Russian forces inched towards Kyiv and pounded civilian areas in other Ukrainian cities Friday, drawing warnings of "unimaginable tragedy" as the United States and the EU moved to tighten the economic noose around the Kremlin.
Sixteen days after Moscow shocked the world by invading Ukraine, the United Nations and others said it may be committing war crimes in cities such as Mariupol, which for days now has been besieged by Vladimir Putin's forces.
On Friday officials in the southern port said more than 1,500 people had been killed during 12 days of attacks.
Survivors have been trying to flee Russian bombardment in a freezing city left without water or heating, and running out of food. The situation is "desperate," a Doctors Without Borders official said.
"Hundreds of thousands of people... are for all intents and purposes besieged," Stephen Cornish, one of those heading the medical charity's Ukraine operation, told AFP in an interview.
"Sieges are a medieval practice that have been outlawed by the modern rules of war for good reason."
As Russia widens its bombardment and talks between Moscow and Kyiv seemingly go nowhere, Ukrainian President Volodymr Zelensky's pleas for NATO to intervene have grown increasingly desperate.
US President Joe Biden on Friday again ruled out direct action against nuclear-armed Russia, warning that it would lead to "World War III."
Instead Washington added more layers of sanctions to those already crippling Russia's economy, this time ending normal trade relations and announcing a ban on signature Russian goods vodka, seafood and diamonds.
The United States and the European Union also suspended the export of their luxury goods to Russia.
"Putin must pay the price. He cannot pursue a war that threatens the very foundation of international peace and stability and then ask for help from the international community," Biden said from the White House.
He spoke as the United Nations said 2.5 million people had now fled Ukraine and around two million more had been internally displaced by the war.
- 'Nobody buries them' -
Yulia, a 29-year-old teacher who fled Mariupol, said her mother-in-law was still there, and told them "the attacks don't stop".
"There are many corpses on the street and nobody buries them," she told AFP.
In a video address released Saturday, Zelensky appealed to the mothers of Russian soldiers to prevent their sons being sent to war.
"I want to say this once again to Russian mothers, especially mothers of conscripts. Do not send your children to war in a foreign country," he said.
In Ukraine's second city Kharkiv, doctors at a hospital described spending two days pumping ash from the stomach of an eight-year-old child whose home was blasted by a Russian missile.
"He still has cinders in his lungs," Dima Kasyanov's doctor told AFP.
Dnipro, an industrial hub of one million inhabitants, saw its image as a relatively safe haven shattered when three missiles hit civilian buildings Friday.
Images of its charred or destroyed buildings -- including a kindergarten with windows blown out -- now join those from Kharkiv and Mariupol as testimony to the brutal conflict.
"Today, we were supposed to host people who need a lot of support," said Svetlana Kalenecheko, who lives and works in a clinic that was damaged.
"Now we can't help anyone."
The attacks on civilians prompted a new flurry of warnings from the Hague and the United Nations Friday that Russia is committing war crimes.
"We are really heading towards an unimaginable tragedy," Cornish, of Doctors Without Borders, warned, insisting that "there is still time to avoid it, and we must see it avoided".
- 'Catastrophe' -
Meanwhile the Kremlin is slowly surrounding Kyiv, with Ukrainian presidential adviser Mykhailo Podolyak calling it a "city under siege".
He tweeted that it was "ready to fight", with checkpoints prepared and supply lines in place, adding: "Kyiv will stand until the end".
The Ukrainian military has said Russia is trying to take out Kyiv's defences to the north and west, where suburbs including Irpin and Bucha have already endured days of heavy bombardment.
Russian armoured vehicles are also advancing on the capital's northeast.
As the slow but steady advance continues, so has the tide of refugees.
Around 100,000 people have been able to leave the northeastern city of Sumy, the eastern city of Izyum, and areas northwest of Kyiv in the last two days, Ukrainian officials said.
Zelensky warned living conditions were deteriorating fast.
"In the Sumy, Kyiv and Donetsk regions, there is no more electricity. Yes, there are problems with heating. There is no gas, no water," he said.
"It's a humanitarian catastrophe."
- 'Murderers from Syria' -
Foreign combatants have already entered the Ukrainian conflict on both sides, and on Friday the Kremlin ramped up efforts to bring in reinforcements, particularly from Syria.
A furious Zelensky accused Russia of hiring "murderers from Syria, a country where everything has been destroyed... like they are doing here to us".
In southern Ukraine, Russian soldiers abducted the mayor of Melitopol, which Zelensky said was a "sign of weakness" and a "crime against democracy".
The global ripple effects of the conflict continued elsewhere.
Last-minute Russian demands related to the conflict threatened to derail the near-complete process of reviving the Iranian nuclear deal Friday.
And the fighting spurred vows to bolster the European Union's defences, with EU leaders describing the invasion as a wake-up call.
"There's no denying the fact that two weeks ago we woke up in a different Europe, in a different world," European Council chief Charles Michel said.
Russia also moved Friday to block Instagram and launch a criminal case against its owner Meta, as Moscow fired back at the tech giant for allowing posts calling for violence against Russian forces.
- 'We will not fight' -
Talks have so far made no progress towards ending the fighting.
Putin has said negotiations were being held "almost daily", and US and European stock markets rose Friday on his comments that there had been "certain positive shifts".
But US Vice President Kamala Harris, speaking in Bucharest, said the Russian leader had shown "no sign of engaging in serious diplomacy".
At the United Nations, Western countries accused Russia of spreading "wild" conspiracy theories after Moscow's envoy told diplomats that America and Ukraine had researched using bats to conduct biological warfare.
The US envoy said Russia had made the claims as part of a "false flag effort" for using chemical weapons of its own in Ukraine.
Biden warned Russia would pay a "severe price" if it used chemical weapons.
But he again carefully steered clear of any indication that such an attack would be a red line that could draw direct US military action.
"We will not fight a war against Russia in Ukraine," he said.
Latest developments
Here are the latest developments in Russia's war in Ukraine:
The capital Kyiv risks being surrounded, with Russian tanks moving in on areas north and west of the capital, the Ukrainian military says, with some suburbs heavily bombarded. Presidential adviser Mykhailo Podolyak calls it a "city under siege".
- West steps up sanctions -
The United States and its allies move to end normal trade relations with Russia, enabling them to inflict steep tariff hikes on Moscow.
Washington announces a ban on Russian seafood, vodka and diamonds, and along with the European Union, bans exports of luxury goods to Russia.
Canada imposes new sanctions on Russian oligarchs including Roman Abramovich.
- No 'World War III' -
US President Joe Biden again rules out any direct intervention by the United States to halt Russia's invasion of Ukraine, warning that such conflict pitting the NATO alliance against the Kremlin "is World War III".
- 'Unimaginable tragedy' -
The situation in Ukraine's southern port city Mariupol, encircled and under constant Russian shelling, is "desperate", a top Doctors Without Borders executive says, calling for immediate action to avoid "unimaginable tragedy".
Mariupol has been without water and power for 11 days.
The city's mayor says it is being shelled "every 30 minutes", with 1,200 civilians already dead and reports of people starving and of corpses in the streets.
- Care home, kindergarten hit -
An apartment block and shoe factory are hit in the first air strikes on the central city of Dnipro -- until now seen as a safe haven -- with a kindergarten also damaged.
A home for the disabled near Kharkiv in the east is also bombed, with 330 people there at the time, officials say.
Russia also announces the military airfields of Lutsk and Ivano-Frankivsk, in western Ukraine closer to the Polish border, have been "put out of action".
- 'Massive' sanctions -
European Union leaders eye an extra 500 million euros for military support for Ukraine and "massive" sanctions on Russia, if the Kremlin presses on with its war.
- Putin slammed for 'hiring' Syria fighters -
The Kremlin says Syrian fighters can fight for Russia in Ukraine after President Vladimir Putin backs plans to draft in 16,000 volunteers, mostly from the Middle East.
Ukraine's President Volodymyr Zelensky accuses Moscow of hiring "murderers" from a country they have destroyed, "like they are doing here to us".
- 100,000 flee -
Around 100,000 people have been able to leave the northeastern city of Sumy, the eastern city of Izyum, and areas northwest of Kyiv in the last two days, Ukrainian officials say.
- Russia restricts Instagram -
Russia restricts access to Instagram, one of the most popular platforms there, claiming it is carrying content urging people to attack its armed forces.
- UN, Moscow accuse Facebook -
It comes after the UN voices alarm at Facebook's decision to temporarily ease its violent speech policy after the invasion, warning it could spark hate speech.
Moscow prosecutors also open a case against its parent company -- which owns Instagram -- for "calling for the murder" of Russians.
The platform says it will allow statements like "death to Russian invaders" but not threats against civilians.
- UN meets on Russia bio-weapon claim -
The UN Security Council will meet Friday after Russia says biological weapons are being developed in Ukraine, claims both Washington and Kyiv deny.
The US says it is a sign Moscow could soon use the weapons itself.
- 2.5 million flee -
More than two and a half million people have fled the "senseless war" in Ukraine, the UN says -- more than half to Poland.