US President Joe Biden wants to significantly ramp up support for Ukraine, asking Congress for US$33 billion ($46.5 billion) as he warned the cost of “caving” to Russia would be much higher.

Mr Biden insisted the US was not “attacking Russia”, but the package of military, economic and humanitarian aid would help Ukraine “defend itself against Russian aggression”.

It comes as Russian President Vladimir Putin earlier warned that any “outside” threats against Moscow would face a “swift, lightning fast” retaliation.

The latest package is almost double what America has so far contributed to the war and includes US$20 billion ($28b) in military aid, US$8.5 billion ($12b) in economic aid and US$3 billion ($4.2b) in humanitarian aid.

“It’s not cheap,” Mr Biden said.

“But caving to aggression is going to be more costly if we allow it to happen.”

Mr Biden also announced plan to liquidate the considerable assets seized from Russian oligarchs and use the proceeds to support Ukraine.

Meanwhile two huge explosions have been heard in Kyiv where UN secretary general António Guterres is to meet with Ukrainian President Volodymyr Zelensky.

Kyiv’s mayor Vitali Klitschko said Russian forces had fired on the Ukrainian capital’s central district.

Smoke rises after an explosion in central Kyiv. Photo: Getty

Earlier Mr Guterres had been surveying the destruction in small towns outside Kyiv and described the war as “evil”.

He condemned the atrocities committed in towns like Bucha, where evidence of mass killings of civilians was found after Russia retreated in the face of unexpectedly stiff resistance.

“Wherever there is a war, the highest price is paid by civilians,” Mr Guterres said as he visited the bombed-out Kyiv suburb of Irpin and reiterated the importance of investigating alleged war crimes.

Russia’s offensive in eastern Ukraine has gathered momentum as several areas come under heavy shelling.

Ukrainian authorities reported intense Russian fire in Donbas and near Kharkiv, a northeastern city outside Donbas that is seen as key to the offensive.

In the ruined southern port of Mariupol, the last Ukrainian fighters holed up in the steel plant said concentrated bombing overnight killed and wounded more people.

UN Secretary-General Antonio Guterres visiting Borodianka, outside Kyiv. Photo: Getty

Authorities warned that a lack of safe drinking water inside Mariupol could lead to outbreaks of deadly diseases.

Authorities said the estimated 100,000 who remain run the risk of diseases like cholera and dysentery.

“Deadly epidemics may break out in the city due to the lack of centralised water supply and sewers,” the council said on Telegram.

It reported bodies decomposing under the rubble and a “catastrophic” shortage of drinking water and food.

Many observers expect Mr Putin will try to claim a big victory in the east by Victory Day, which marks the defeat of Nazi Germany during World War II.

Bulgarian Prime Minister Kiril Petkov vowed his country would join others in providing military assistance as he toured another scene of atrocities outside Kyiv, in Borodyanka.

“We cannot be indifferent. We cannot say that this is a Ukrainian problem. We cannot say some people are dying but we are not interested in that,” he said.

“This is not just the battle for Ukraine but it is a matter for civilisation to choose which side to take.”

The visit by the Bulgarian leader came a day after Russia cut off the supply of natural gas to his country and fellow NATO member Poland, in what was seen as a bid to punish and divide the continent.

Meanwhile in Russia itself, witnesses have reported two powerful blasts in the city of Belgorod, near the border with Ukraine.

Russia has in recent days reported what it says are a series of attacks by Ukrainian forces on Russian regions which border Ukraine, and has warned that such attacks raise a risk of significant escalation.

Ukraine has not directly accepted responsibility but has described the incidents as payback and “karma” for Russia.

-with AAP