President Joe Biden for the first time Tuesday accused Vladimir Putin's forces of committing genocide in Ukraine, where Russia was intensifying its campaign to subdue the devastated port city of Mariupol.

Biden's accusation came as Moscow -- already accused by the West of widespread atrocities against civilians -- was feared to be readying a massive onslaught across Ukraine's east that Washington warned might involve chemical weapons.

"Yes, I called it genocide," Biden told reporters, hours after employing the term during a speech in Iowa -- its first use by a member of his administration.

"We'll let the lawyers decide internationally whether or not it qualifies, but it sure seems that way to me," Biden said. "It's become clearer and clearer that Putin is just trying to wipe out the idea of even being able to be a Ukrainian."

Ukrainian President Volodymyr Zelensky -- who has repeatedly accused Moscow of attempted "genocide" -- swiftly responded by tweeting at Biden: "True words of a true leader."

"Calling things by their names is essential to stand up to evil," Zelensky wrote -- renewing his appeal for more heavy weapons to "prevent further Russian atrocities."

Biden had previously described Putin as a "war criminal" as the discovery of hundreds of civilians reportedly killed in Bucha, outside Kyiv, sparked global revulsion.

But he had stopped short of using the term "genocide," in line with longstanding US protocol, because of its strict legal definition and the heavy implication the accusation carries. 

Adding to the catalogue of horrors emerging from Ukraine, Zelensky sounded the alarm Tuesday about snowballing allegations of rape and sexual assault by Russian forces.

"Hundreds of cases of rape have been recorded, including those of young girls and very young children. Even of a baby!" the Ukrainian leader told Lithuanian lawmakers via video link.

- Chemical weapon fears -

In the latest discovery fuelling allegations of Russian atrocities, Ukrainian prosecutors said six people had been found shot dead in the basement of a building outside the capital. 

While the toll on towns occupied during the month-long offensive to take Kyiv is still coming to light, the heaviest civilian toll is feared to be in Mariupol, where Zelensky said he believed Russia had killed "tens of thousands."

AFP journalists in Mariupol, as part of a Russian military embed, witnessed the charred remains of the city, including the theatre where 300 people were feared killed in Russian bombardment last month.

As fighting dragged toward its seventh week, the Ukrainian army was fighting desperately to defend strategically located Mariupol.

Moscow is believed to be trying to connect occupied Crimea with Russian-backed separatist territories Donetsk and Lugansk in Donbas, and has laid siege to the city, once home to more than 400,000 people.

Reports emerged on Monday from Ukraine's Azov battalion that a Russian drone had dropped a "poisonous substance" in the area, with people experiencing respiratory failure and neurological problems.

US Secretary of State Antony Blinken said he was unable to confirm the allegations, but that Washington had "credible information" Russia might use tear gas mixed with chemical agents in the besieged port.

The world's chemical weapons watchdog said it was "concerned" by the unconfirmed reports coming from Mariupol, and was "monitoring closely."

Pentagon spokesman John Kirby warned the use of such weapons by Moscow would "elicit a response not just from the United States, but from the international community," without elaborating.

- 'They will remember' -

With little hope of a quick end to fighting, President Vladimir Putin pledged Moscow would proceed on its own timetable, rebuffing repeated international calls for a ceasefire.

"Our task is to fulfil and achieve all the goals set, minimising losses. And we will act rhythmically, calmly, according to the plan originally proposed by the General Staff," Putin told a news conference with Belarusian President Alexander Lukashenko.

He also dismissed as "fake" claims that hundreds of civilians were killed in Bucha under Russian occupation.

Bucha Mayor Anatoly Fedoruk said more than 400 people had been found dead after Moscow's forces withdrew, and 25 women reported being raped, as the town prepares for the return of residents who fled the fighting.

"What people will find in their homes is shocking, and they will remember the Russian occupiers for a very long time," he said.

- 'Devil incarnate' -

Heavy bombardment continued in Ukraine's east as civilians were urged to flee ahead of an expected Russian troop surge around the Donbas region, notably near the town of Izyum -- adding to the 10 million people already displaced by fighting.

A steady stream of residents fled by bus and train from Kramatorsk -- the Ukrainian military's main hub for its operations in the east -- and neighbouring Sloviansk as fears grew that the cities would be key targets.

"What is happening is inhuman, (Putin) is a fascist. I don't know what to call him -- a devil incarnate," said 82-year-old Valentina Oleynikova, who was fleeing Kramatorsk with her husband.

In the war-torn eastern town of Volnovakha, now under Moscow's control, a school reopened with children listening to a recording of the Russian anthem, watched by armed soldiers.

After two weeks of bombardment, many houses, shops and public buildings are now semi-ruined, windowless or burnt out.

- Tycoon swap -

In a separate development, Zelensky offered to swap a pro-Kremlin tycoon -- arrested after escaping from house arrest -- for Ukrainians captured by Russia.

Zelensky posted a picture of a dishevelled-looking Viktor Medvedchuk -- one of the richest people in Ukraine, who counts Putin among his personal friends -- with his hands in cuffs and dressed in a Ukrainian army uniform.

"I propose to the Russian Federation to exchange this guy of yours for our boys and our girls who are now in Russian captivity," Zelensky said in a video address on Telegram.

Medvedchuk, a hugely controversial figure in Ukraine, was under house arrest over accusations of attempting to steal natural resources from Russia-annexed Crimea and of handing Ukrainian military secrets to Moscow. 

Latest developments

Here are the latest developments in the war in Ukraine:

- Biden accuses Russians of 'genocide' -

US President Joe Biden accuses Russian forces of committing genocide in Ukraine, the first time his administration has used the term.

"Yes, I called it genocide," Biden tells reporters travelling with him in Iowa when asked about his use of the term during an earlier speech. 

"It's become clearer and clearer that Putin is just trying to wipe out the idea of even being able to be a Ukrainian." 

Ukrainian President Volodymyr Zelensky swiftly responds: "True words of a true leader."

- Ukrainians 'surrounded' in Mariupol -

Ukrainian forces are "surrounded and blocked" in Mariupol as Russian forces push to take the southeastern port city, Mykhaylo Podolyak, an official from Zelensky's office, tweets.

Zelensky says he believes "tens of thousands" of people in the city have been killed and makes another plea for weapons. 

- 'Credible information' on chemical weapons -

The United States has "credible information" that Russia "may use... chemical agents" in its offensive to take Mariupol, US Secretary of State Antony Blinken says.

He tells reporters he is not able to confirm accusations that Moscow has already used chemical weapons there.

The world's chemical weapons watchdog, the OPCW, says it is "concerned" over reports of the use of chemical weapons in Mariupol.

- 'All options on table' -

Britain's armed forces minister James Heappey tells Sky News that if evidence of chemical weapons use emerges, "all options are on the table" as a response.

"There are some things that are beyond the pale, and the use of chemical weapons will get a response," he says.

- Over 400 bodies in Bucha - 

The mayor of the town of Bucha, where dozens of bodies were found after Russia's withdrawal from northern Ukraine, says more than 400 people have been found dead so far and 25 women have reported being raped.

Zelensky says investigators have received reports of "hundreds of cases of rape" in areas previously occupied by Russian troops, including sexual assaults of small children.

- Burials in east -

Around 400 civilians have been buried in the town of Severodonetsk near the frontline in eastern Ukraine since the Russian invasion, the governor of the Lugansk region, Sergiy Gaiday, says.

- Rape allegations -

Zelensky says hundreds of rapes by Russian forces have been recorded, including of very young children and "even of a baby". 

- Invasion going 'calmly': Putin -

Putin says Russia's offensive is proceeding "calmly" and according to plan, with the goal of "minimising losses".

During a televised press conference with Belarusian President Alexander Lukashenko, Putin dismisses reports of the discovery of hundreds of bodies of civilians in Bucha as fake.

- Residents flee east -

Residents stream out of east Ukraine's Kramatorsk and Sloviansk as fears grow the cities will be key targets of a major new Russian offensive. 

The Pentagon says Russia is building up its forces in the eastern Donbas region, as it switches its focus to a region where pro-Russian rebels have been fighting Ukrainian forces since 2014. 

- Talks 'extremely difficult': Kyiv -

Kyiv says talks with Russia to end the war are "extremely difficult".

"Negotiations are extremely difficult," Ukrainian presidential adviser Podolyak says.

Putin says Ukraine's "inconsistency on fundamental points" is creating "certain difficulties in reaching final agreements".

- German president 'not wanted' in Kyiv -

German President Frank-Walter Steinmeier says he offered to visit Ukraine with other EU leaders but was told by Kyiv his trip was "not wanted".

Steinmeier, a former foreign minister under ex-chancellor Angela Merkel, was long known for championing ties with Moscow. The snub comes as Chancellor Olaf Scholz is under increasing pressure for not having visited Ukraine.

- Tycoon swap offer -

Zelensky offers to swap pro-Kremlin tycoon Viktor Medvedchuk, who was arrested after escaping from house arrest, for Ukrainians captured by Russia.

- Over 870,000 returnees -

More than 870,000 Ukrainians who fled abroad since the start of the war have returned to the country, Ukraine's border force says. 

Spokesman Andriy Demchenko says that 25,000 to 30,000 Ukrainians are returning each day, with growing numbers of women, children and elderly among them.

In total, more than 4.6 million Ukrainians have now fled their country, the United Nations says.