Biden accuses Putin's forces of 'genocide' in Ukraine

US President Joe Biden has for the first time accused Vladimir Putin's forces of committing genocide in Ukraine, where Russia is intensifying its campaign to take the strategic port city of Mariupol.

Biden's accusation comes as Moscow -- already accused by the West of widespread atrocities against civilians -- is 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 on Tuesday, hours after employing the term during a speech in Iowa -- its first use by a member of his administration, reports AFP.

"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."

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.

- Tunnel warriors -

While the toll on towns occupied during the month-long offensive to take Kyiv like Bucha 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".

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 strategically located city.

Experts say its fall is inevitable, but as fighting drags toward its seventh week, the Ukrainian army is still clinging on.

On Wednesday the Land Forces of Ukraine said on Telegram that air strikes on the city continued, particularly targeting its port and the huge Azovstal iron and steel works.

The latter maze-like complex has been a focus of urban resistance in Mariupol, with fighters using a tunnel system below the vast industrial site to slow Russian forces down.

"It's a city within a city," said Eduard Basurin, a representative for pro-Russian separatists in the eastern Donetsk region.

"There are several underground levels that date back to Soviet times which you can't bombard from above. You have to go underground to clean them out, and that will take time."

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

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.

- 'Devil incarnate' -

Heavy bombardment continued across the east as civilians were urged to flee ahead of an expected Russian troop surge around the Donbas region, notably near the town of Izyum.

US private satellite firm Maxar Technologies published images it said showed ground forces moving towards Russia's border with Ukraine, likely in preparation for an offensive.

In Ukraine, Maxar said it had noted convoys of military equipment travelling in and near the Donbas region -- adding they comprised of around 200 vehicles including tanks, artillery and armoured personnel carriers.

Heeding the calls from authorities to flee, a steady stream of residents left by bus and train from the cities of Kramatorsk and neighbouring Sloviansk.

Kramatorsk is the Ukrainian military's main hub for its operations in the east, and so potentially a key target.

"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 the city with her husband.

With little hope of a quick end to fighting, 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," he told a news conference with Belarusian President Alexander Lukashenko.

- 'They will remember' -

Putin 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.

In nearby Gostomel, war crimes investigators were beginning a grim probe, exhuming bodies to document the cause of death.

One of those was that of the mayor, who the council said was "handing out bread to the hungry" when he was shot by Russian forces.

His is among the fates that are known.

"The town council has counted the number of missing at up to 400," said regional prosecutor Andriy Tkach. "Perhaps not all the bodies are found."

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.

- Tycoon swap -

In a separate development, Zelensky has 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.