Some 7.7 million Ukrainian refugees abroad are being urged not to return to their homeland before spring as the country struggles through a harsh winter without enough power.

A wave of Russian attacks targeting the power network destroyed about one-third of Ukraine’s energy sector.

Deputy Prime Minister Iryna Vereshchuk said Ukrainians would have to try and “survive the winter” and that “the situation will only get worse”.

“The networks will not cope,” Ms Vereshchuk said.

“You see what Russia is doing. If it is possible, stay abroad for the time being.”

The UN’s refugee agency says there are about 7.7 million refugees from Ukraine across Europe, including Russia, from a population of about 44 million.

About 7.7 million Ukrainians have fled their homeland during the Russian invasion. Photo: Getty

Russia makes  ‘dirty bomb’ move

Meanwhile Russia has taken its accusation that Ukraine is preparing to use a dirty bomb to a closed meeting of the United Nations Security Council.

Russia alleges Ukraine has ordered two organisations to create a dirty bomb – an explosive device laced with radioactive material – without giving any evidence.

There is growing concern that Russia is using the claims as a pretext to escalate the war as it struggles to hold ground.

“I don’t mind people saying that Russia is crying wolf if this doesn’t happen because this is a terrible, terrible disaster that threatens potentially the whole of the earth,” said Russia’s deputy UN ambassador Dmitry Polyanskiy.

When asked what evidence Russia had to back its claims, Mr Polyanskiy said it was intelligence information that had been shared with foreign counterparts with the “necessary level of clearance”.

The UN nuclear watchdog – the International Atomic Energy Agency – is preparing to send inspectors soon to two Ukrainian sites at the request of Kyiv officials – an apparent reaction to Russia’s dirty bomb claims.

Ukrainian President Volodymyr Zelensky has responded by accusing Russia of planning such an attack itself to blame on Ukraine.

“If Russia calls and says that Ukraine is allegedly preparing something, it means one thing: Russia has already prepared all this,” Mr Zelensky said.

Ukraine’s allies also rejected Russia’s allegation as a pretext for intensifying the eight-month-long war.

Kremlin spokesman Dmitry Peskov on Tuesday repeated Russia’s allegations and said the US and its allies were foolish to dismiss them.

Russia sent a letter on its assertion about Kyiv to the UN late on Monday, and diplomats said Moscow planned to raise the issue with the Security Council at a closed meeting on Tuesday.

“We will regard the use of the dirty bomb by the Kyiv regime as an act of nuclear terrorism,” Russia’s UN Ambassador Vassily Nebenzia told UN Secretary-General Antonio Guterres and the Security Council in the letter, seen by Reuters.

With Ukrainian forces advancing into Russian-occupied Kherson province, top Russian officials had phoned their Western counterparts on Sunday and Monday to air their suspicions.

The foreign ministers of France, Britain and the US rejected Moscow’s allegations as “transparently false” and reiterated their support for Ukraine.

“The world would see through any attempt to use this allegation as a pretext for escalation,” they said in a joint statement.

Later, the US issued a warning to Russia.

“We’ve been very clear with the Russians … about the severe consequences that would result from nuclear use,” State Department spokesman Ned Price said.

“There would be consequences for Russia whether it uses a dirty bomb or a nuclear bomb.”

Russia’s defence ministry said the aim of a “dirty bomb” attack by Ukraine would be to blame Russia for the resulting radioactive contamination.

The ministry has begun preparing for such a scenario, it said, readying forces and resources “to perform tasks in conditions of radioactive contamination.”

Russia’s state news agency RIA had earlier identified what it said were the two sites involved in the operation – the Eastern Mineral Enrichment Plant in central Dnipropetrovsk region and the Institute for Nuclear Research in Kyiv.

US officials said there was no indication Russia had decided to use a dirty bomb or any nuclear weapon.

“We continue to see nothing in the way of preparations by the Russian side for the use of nuclear weapons,” White House national security spokesman John Kirby told reporters.

Ukrainian foreign minister Dmytro Kuleba said late Monday he had a discussion with US Secretary of State Antony Blinken on “ways to end Russia’s nuclear blackmail”.

-with AAP