US President Joe Biden has lashed Vladimir Putin as a “war criminal” after Russia blasted a theatre and a swimming pool where hundreds of civilians were sheltering in the embattled Ukraine city of Mariupol.

Up to 1200 people, including hundreds of children and elderly people, were thought to have been hiding in the theatre when it was targeted in a Russian attack on Thursday.

The number of casualties is not yet known.

“According to preliminary data, several hundred Mariupol residents were hiding in the drama theatre. Their fate is unknown, as the entrance to the bomb shelter is blocked by rubble,” Donetsk regional administration head Pavlo Kyrylenko said.

The US company Maxar released satellite pictures taken on March 14, which it said showed the word “children” had been written in Russian on the concrete outside the building.

Mr Kyrylenko said Russian forces were trying to “physically destroy Mariupol and the people of Mariupol, which have been a symbol of our resistance”.

Later on Thursday, another bomb hit a swimming pool about four kilometres north of the theatre.

Maxim Kach, a Mariupol city government official, said​ rescue workers were trying to get a pregnant woman out from under the rubble.

“Here there were only pregnant women and women with kids under three years old,” he said in a video message.

Mr Kach said there were no military ​personnel at, or near, the pool.

The word “children” is painted in Russian outside the theatre. Photo: Getty

Ukraine President Volodymyr Zelensky accused Russia of crossing “all the red lines” after the attacks. In an interview with US network NBC after his earlier address to Congress on Thursday, Mr Zelensky was asked if a chemical attack by Russia would be a red line that would prompt the US to become more actively involved in combat.

“I believe that Russians have already crossed all the red lines when they started shelling civilians,” he said.

Mr Zelensky said Russia had already killed more than 100 children in Ukraine – even before the attacks on the theatre and pool.

“I don’t understand the meaning of red lines. What else should we wait for? For letting Russians kill 200, 300 or 400 children?”

Earlier, after Mr Zelensky’s impassioned plea to the US Congress for a no-fly zone and more sanctions, Mr Biden said it would send $US800 million ($A1.1 billion) in weapons to Ukraine.

“I think he is a war criminal,” Mr Biden said of the Russian President in a dramatic shift from his administration’s previous stance.

White House officials, including Mr Biden, had previously stopped short of saying war crimes were being committed in Ukraine.

White House press secretary Jen Psaki said the US State Department was investigating whether to designate Mr Putin as a war criminal.

“The President’s remarks speak for themselves. He was speaking from his heart and speaking from what he’s seen on television, which is barbaric actions by a brutal dictator through an invasion of a foreign country,” she said.

The Kremlin swiftly lashed Mr Biden’s comments as “unacceptable and unforgivable rhetoric”.

Nowhere safe from Russia’s wrath: Canberra envoy

If Ukraine falls, further countries will face the wrath of Russian expansionism, Ukraine’s top diplomat in Canberra warned as he reiterated Mr Zelensky’s call for more aid.

Charge d’Affaires Volodymyr Shalkivskyi said Ukraine needed more aircraft, modern anti-aircraft systems and a no-fly zone in its skies to stave off the Russian advance as troops closed in on Kyiv.

“[Russia’s] ally Belarus needs access to the Baltic Sea and [needs] to go through the European Union and NATO members, so there is already clear evidence that in case Ukraine falls, then there will be next countries in line,” he told the Seven Network.

“Our President is calling for the active participation of the world community and helping Ukraine to protect our land.”

Mr Shalkivskyi said Ukraine needed defensive systems and lethal support “in order for us to effectively sustain that military pressure that’s coming from Russia”.

“Russia has air superiority and the devastation that it caused on the ground in terms of civilian death and destruction of residential areas and civilian infrastructure is just striking,” he said.

Mr Shalkivskyi said Russia had shown signs it was committing genocide by targeting civilians and attacking humanitarian corridors when civilians were trying to flee conflict zones.

“These mass killings and slaughters continue and we are calling for international partners to step in,” he said.

Australia has flagged it could send more military aid, with Prime Minister Scott Morrison saying this may mean more armaments with international partners. A further round of sanctions is also on the cards.

The federal government reportedly has its eye on two billionaires – Oleg Deripaska and Viktor Vekselberg – with links to Russia’s Rusal, one of the world’s biggest aluminium manufacturers.

A spokesperson for Foreign Minister Marise Payne said she was awaiting advice from the Department of Foreign Affairs and Trade on further sanctions.

“We are consulting and cooperating closely with partners on sanctions, and would note that the UK only sanctioned Viktor Vekselberg and Oleg Deripaska in recent days,” the spokesperson said.

-with AAP