Boris Johnson has accused Vladimir Putin of “directly threatening the safety of all of Europe” after Russia appeared to attack a nuclear power plant in Ukraine.
The prime minister held an emergency call with Ukrainian president Volodymyr Zelensky following the incident at the Zaporizhzhia station.
Both leaders agreed that Russia “must immediately cease its attack on the power station and allow unfettered access for emergency services ”.
A fire broke out in a training building outside the facility — Europe’s largest nuclear power plant — during intense fighting between Russian and Ukrainian forces.
Johnson is calling for a meeting of the UN Security Council, of which Russia is a member, to discuss the international response to the crisis.
US energy secretary Jennifer Granholm said there was no indication of elevated radiation levels at the Zaporizhzhia plant, which provides more than a fifth of total electricity generated in Ukraine.
Ukrainian foreign minister Dmytro Kuleba said the Russian army was “firing from all sides” on the plant.
“Fire has already broke out ... Russians must IMMEDIATELY cease the fire, allow firefighters, establish a security zone!,” he wrote on Twitter.
A Downing Street spokeswoman said: “The prime minister spoke to Ukrainian President Zelenskyy in the early hours of this morning about the gravely concerning situation at Zaporizhzhia nuclear power station.
“Both leaders agreed that Russia must immediately cease its attack on the power station and allow unfettered access for emergency services to the plant.
“The prime minister said the reckless actions of president Putin could now directly threaten the safety of all of Europe. He said the UK would do everything it could to ensure the situation did not deteriorate further.
“The prime minister said he would be seeking an emergency UN Security Council meeting in the coming hours, and that the UK would raise this issue immediately with Russia and close partners.
“Both leaders agreed a ceasefire was crucial.”
As the biggest attack on a European state since World War Two 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.
Deputy prime minister Dominic Raab also condemned the “reckless” attack on the nuclear power plant and said it was “doubly reprehensible” that Russian forces had continued to shell the site after Ukrainian emergency services had tried to put the fire out.
Raab said the international community needed to “come down hard on Putin” as a result.
“It is clearly reckless, irresponsible and not only the fact they were shooting, bombarding that particular site, but when the Ukrainian emergency authorities were trying to put out the fire, the shelling continued,” he told Times Radio.
“We support the Ukrainians in dealing with the security situation there but also I think come down hard on Vladimir Putin.
“That’s why the Prime Minister has called for an emergency United Nations security council meeting in New York so the entire international community can address this, because of course it is a much wider threat, given the nuclear implications.
“It is an affront to the world at large.”