Russia has taken control of a nuclear power station in Ukraine after it was hit by shelling.
A fire broke out at the Zaporizhzhia nuclear plant - the largest in Europe - and Ukraine said it was shelled by Russian troops, reports BBC.
Authorities say the facility is now safe and radiation levels are normal.
World leaders have accused Russia of endangering the safety of an entire continent, and Ukraine's president accused Russia of "nuclear terror".
US President Joe Biden urged Moscow to stop its military activities around the site, while Canada's Prime Minister Justin Trudeau said the "horrific attacks" from Russia "must cease immediately".
UK Prime Minister Boris Johnson said the "reckless" attack could "directly threaten the safety of all of Europe".
All three leaders spoke to Ukraine's President Volodymyr Zelensky by phone.
Meanwhile, Zelensky said Russia wanted a repeat of Chernobyl, the site of the world's worst nuclear disaster in 1986.
"If there is an explosion, it is the end of everything. The end of Europe," he said.
Russia's defence ministry blamed the attack on Ukrainian saboteurs, calling it a "monstrous provocation" without providing evidence.
A video feed from the nuclear plant showed blasts lighting up the night sky and sending up plumes of smoke.
Buildings around one of the plant's six power units have been damaged without affecting its safety, according to Ukraine's nuclear inspectorate.
Workers at the plant said the fire - which has since been extinguished - broke out at a training building outside the plant's perimeter, and that only one of the plant's six reactors was operational.
One resident who lives nearby said he saw the Russian military attacking the site. "It is just terrorism... It is worrying not just for our region, but for Ukraine and for the world," Kirill Dovzhik told the BBC.
The UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), said the fire had not affected the plant's "essential" equipment and there was no increase in radiation levels.
However, the IAEA said it was in "full 24/7 response mode" due to the "serious situation" at the power station.
Ukrainian emergency services said they were initially blocked from getting to the scene of the blaze, prompting President Biden to publicly call on Russia to allow firefighters into the site.
Boris Johnson said he would seek an emergency meeting of the UN Security Council on Friday over the attack.