Leaders of the G7 have vowed to hold Vladimir Putin to account for Russia’s revenge bombardment of Ukraine and will back the besieged nation for “as long as it takes”.

In an emergency virtual meeting, the rich nations accused Moscow of “war crimes” as missiles continued to fall on Ukraine cities, killing civilians and destroying vital infrastructure.

Russia’s biggest aerial assaults since the beginning of the war have drawn widespread international condemnation and prompted Ukraine to plead for more air defences.

The G7 warned that “any use of chemical, biological or nuclear weapons by Russia would be met with severe consequences”.

“We condemn these attacks in the strongest possible terms and recall that indiscriminate attacks on innocent civilian populations constitute a war crime,” said the G7 statement.

“We will hold President Putin and those responsible to account.”

The G7, which includes the USA, promised to stock up Ukraine’s supply of weapons as well as other humanitarian support.

“We reassured President Zelensky that we are undeterred and steadfast in our commitment to providing the support Ukraine needs to uphold its sovereignty and territorial integrity.

“We will continue to provide financial, humanitarian, military, diplomatic and legal support and will stand firmly with Ukraine for as long as it takes.”

It comes as NATO also said it would stand with Ukraine for as long as necessary and was closely watching Russia’s nuclear forces.

NATO Secretary General Jens Stoltenberg said NATO would “step up” support for Ukraine and was keeping an eye on the growing nuclear threat posed by Russia.

“We are closely monitoring Russia’s nuclear forces; we have not seen any changes in Russia’s posture, but we remain vigilant,” he said.

France’s President Emmanuel Macron joined the G7 virtual meeting. Photo: Getty

Russia has launched fresh rockets and combat drones over several regions of Ukraine following heavy bombardment the day before, taking out powerplants ahead of winter.

So far about 100 missiles have been counted, most of them on Monday (local time) when Moscow launched the attacks in retaliation for the partial destruction of the bridge to Crimea.

According to Ukraine’s government, 19 people have died and more than 100 were injured across the country.

Hundreds of localities were still without electricity as a result of Monday’s bombardments, Ukrainian authorities said.

At one point, more than 3500 communities were in the dark.

Hundreds of emergency workers have fanned out across the country to clean up from the aftermath of the Russian attacks.

Poland said it was preparing for the possibility of a new wave of war refugees from Ukraine following the intensification of Russian attacks.

“We are ready at any time, our borders are open, and we are also logistically prepared to receive refugees,” Marlena Malag, the family and social policy minister, told told public broadcaster TVP.

Poland and Ukraine share a border more than 500 kilometres long.

Russia ‘open for talks’

Meanwhile Russian Foreign Minister Sergei Lavrov says Moscow is open to talks with the West on the Ukraine war but has yet to receive any serious proposal to negotiate.

In an interview on state TV, Lavrov said officials including White House national security spokesman John Kirby had said the United States was open to talks but that Russia had refused.

“This is a lie,” Mr Lavrov said. “We have not received any serious offers to make contact.”

He also said Russia would not turn down a meeting between President Vladimir Putin and US President Joe Biden at a forthcoming G20 meeting and would consider the proposal if it receives one.

“We have repeatedly said that we never refuse meetings. If there is a proposal, then we will consider it,” Mr Lavrov said.

Russia has warned the West about its growing involvement in the Ukraine conflict but said direct conflict with NATO was not in Moscow’s interests.

“We warn and hope that they realise the danger of uncontrolled escalation in Washington and other Western capitals,” Russian Deputy Foreign Minister Sergei Ryabkov was quoted as saying by RIA news agency on Tuesday.

-with AAP