A building outside the largest nuclear power plant in Europe caught fire during intense fighting between Russian and Ukrainian forces on Friday, Ukraine’s state emergency service says.

A video feed from the plant, about 550 kilometres, south-east of the capital Kyiv appeared to show smoke and flames coming from an unidentified building.

Unconfirmed reports said it was an administrative building at the Zaporizhzhia plant that was burning.

“Russian army is firing from all sides upon Zaporizhzhia NPP, the largest nuclear power plant in Europe,” Foreign Minister Dmytro Kuleba wrote on Twitter.

Earlier, Energodar mayor Dmitry Orlov reported fierce fighting between local forces and Russian troops in the area, which is about 550 kilometres south-east of Kyiv. He said there had been casualties, but gave no further details.

Tweet from @MarisePayne

Ukrainian authorities also reported Russian troops were stepping up efforts to seize the plant and had entered the town with tanks. They initially said they were prevented from fighting the blaze because of the attacks.

“A threat to world security!!! As a result of relentless shelling by the enemy of the buildings and blocks of the largest nuclear power plant in Europe, the Zaporizhzhia nuclear power plant is on fire!!!” Mr Orlov posted to Facebook.

“I demand, stop! Immediately stop shelling the Zaporizhzhya nuclear power plant point blank.”

As news of the attack emerged on Friday (Australian time), Ukraine’s Foreign Minister Dmytro Kuleba called for an immediate ceasefire, warning that if the plant exploded, it would be “10 times larger than Chernobyl”.

“Russians must immediately cease the fire, allow firefighters, establish a security zone!” Mr Kuleba wrote on Twitter.

Ukrainian President Volodymyr Zelensky accused Russia of resorting to “nuclear terror” and wanting to repeat the Chernobyl disaster of 1986.

“You know the word ‘Chernobyl’,” he wrote on Twitter.

“No country other than Russia has ever fired on nuclear power units,” Mr Zelensky said in a later video message.

“This is the first time in our history. In the history of mankind.”

A White House official said it was monitoring the Zaporizhzhia nuclear power plant. US President Joe Biden reportedly spoke to Mr Zelensky as news of the blaze emerged.

“President Biden joined President Zelensky in urging Russia to cease its military activities in the area and allow firefighters and emergency responders to access the site,” the White House said.

Tweet from @WhiteHouse

In Britain, Prime Minister Boris Johnson accused Russian President Vladimir Putin of endangering all of Europe with the attack on the power plant.

“The Prime Minister said the reckless actions of President Putin could now directly threaten the safety of all of Europe,” a Downing Street spokesman said.

Russia has already captured the defunct Chernobyl plant, some 100 kilometres north of Ukraine’s capital Kyiv.

The International Atomic Energy Agency said in a tweet that it was “aware of reports of shelling” at the power plant and was in contact with Ukrainian authorities.

Later, the US Energy Secretary, Jennifer Granholm, said Zaporizhzhia was “protected by robust containment structures and reactors are being safely shut down”.

A spokesperson for the Zaporizhzhia nuclear power plant said later that background levels of radiation had not changed, while the plant’s director told Ukraine TV that radiation security had been secured.

However, early reports of the incident at the power plant sent financial markets in Asia spiralling, with stocks tumbling and oil prices surging further.

Tweet from @DmytroKuleba

As the biggest attack on a European state since World War II enters its ninth day, thousands are thought to have died or been wounded, one million refugees have fled Ukraine and Russia’s economy has been rocked by international sanctions.

On Thursday, the US and Britain announced sanctions on more Russian oligarchs, following on from EU measures, as they ratcheted up the pressure on the Kremlin.

Sanctions had “had a profound impact already”, Mr Biden said.

Russia calls its actions in Ukraine a “special operation” that is not designed to occupy territory but to destroy its neighbour’s military capabilities and capture what it regards as dangerous nationalists. It denies targeting civilians.

Russia and Ukraine earlier agreed to the need for humanitarian corridors to help civilians escape Moscow’s eight-day-long invasion, the first apparent progress in talks, as the US and Britain hit more oligarchs with sanctions.

Russian forces, however, have continued to surround and attack Ukrainian cities. They include Mariupol, the main port in the east, which has been under heavy bombardment, with no water or power. Officials say they cannot evacuate the wounded.

After talks at an undisclosed location, Russia said “substantial progress” had been made while the Ukrainian side pointed to an understanding on helping ordinary people, but not the results Kyiv had hoped for.

Ukrainian presidential adviser Mykhailo Podolyak said a temporary halt to fighting in select locations was also possible.

“That is, not everywhere, but only in those places where the humanitarian corridors themselves will be located, it will be possible to cease fire for the duration of the evacuation,” he said.

They had also seen eye-to-eye on the delivery of medicines and food to the places where the fiercest fighting was taking place. The negotiators will meet again next week, the Belarusian state news agency Belta quoted Mr Podolyak as saying.

-with AAP