Russia closes on Mariupol as US warns of chemical threat

Russian troops intensified their campaign to take the port city of Mariupol on Tuesday as part of an anticipated massive onslaught across eastern Ukraine that the United States warned might include the use of chemical weapons.

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, 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 United States sounded the alarm as civilians were struggling to flee, with Ukrainian President Volodymyr Zelensky condemning alleged mass rapes in areas previously occupied by Russian troops.

The last time chemical weapons were unleashed during a conflict was in Syria where a civil war erupted in 2011 as rebels sought to oust President Bashar al-Assad.

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

"We have had justified reason to be concerned... that this could be a tactic they might employ, which is to try to mask a potential more serious chemical attack with the riot control agents," he told reporters.

As the fighting dragged toward its seventh week, the Ukrainian army fought desperately to defend Mariupol against the Russian offensive.

'They will remember'

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.

In his nightly address, Zelensky said he believed Russia had killed "at least tens of thousands of people" in the city.

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 dismissed as "fake" claims that hundreds of civilians were killed in the town of Bucha outside the Ukrainian capital Kyiv after the withdrawal of Moscow's forces.

Images taken by journalists on the ground, including AFP reporters, of bodies littering the streets of Bucha sparked worldwide outrage and calls for an investigation into possible war crimes.

Bucha Mayor Anatoly Fedoruk said more than 400 people had been found dead 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 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.

"What we are doing is helping people -- rescuing them on the one hand and on the other taking measures to assure Russia's security," Putin said.

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.

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.

Meanwhile, the toll on towns previously occupied by Russian forces during their month-long offensive to take Kyiv is still coming to light.

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

'We have nothing'

UN officials called for a probe into repeated accounts of rapes and sexual assaults against women that have angered Zelensky.

"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.

More than 4.6 million Ukrainian refugees have now fled the country, the UN refugee agency said - 90 percent of them women and children.

The war has displaced more than 10 million people overall.

One of them was Tatyana Kaftan, just weeks away from giving birth to her first child, who spoke to AFP at an aid distribution point in the western city of Lviv.

Her husband, who is waiting to be called up to the army, stood by her side.

"We left everything at home," said the 35-year-old travel agent, who drove with her husband all the way from Mykolaiv to escape Russian shelling.

"We have nothing."