Prime Minister Boris Johnson has said Russia's shelling of a nuclear power station in the south-eastern Ukrainian city of Zaporizhzhia could "directly threaten the safety of all of Europe".
The power station - the largest in Europe - caught fire after it was attacked by Russia.
While the reactor is under renovation, according to the power plant's spokesman, it still contains nuclear fuel.
In an early morning phone call with Ukrainian President Zelensky, Mr Johnson vowed to seek an emergency UN Security Council meeting in a bid for a ceasefire.
Downing Street called the situation in Zaporizhzhia "gravely concerning".
In a statement sent to PA News Agency, a spokeswoman for the PM's office said: "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."
Several hours later, Ukrainian emergency services announced on social media that the fire had been extinguished.
Firefighters added that the blaze had been in the educational and training building of the plant and no one was harmed.
Earlier in the night, the plant's spokesman Andriy Tuz told Ukrainian television that shells were falling directly on the Zaporizhzhia plant and had set fire to one of the facility's six reactors.
Firefighters could not get near the fire because they were being shot at, Mr Tuz said.
According to the International Atomic Energy Agency (IAEA) the fire did not cause a change in the plant's radiation level. Nor had the shelling or fire caused any damage to "essential" equipment.
In a series of statements to posted to Twitter, the agency said: "Ukraine regulator tells IAEA there has been no change reported in radiation levels at the Zaporizhzhia Nuclear Power Plant site.
"Ukraine tells IAEA that fire at site of Zaporizhzhia Nuclear Power Plant has not affected 'essential' equipment, plant personnel taking mitigatory actions."
The agency added that its Director General Mariano Grossi was in touch with Ukraine's Prime Minister Denys Schmygal and the Ukrainian regulator and operator about the situation.
In another tweet, the IAEA said: Grossi "appeals for halt of use of force and warns of severe danger if reactors hit'.
The agency later tweeted to say it had put its incident and emergency centre in "full 24/7 response mode due to serious situation at Zaporizhzhia".
It comes ahead of crisis talks between Western ministers as Vladimir Putin steps up his assault on Ukraine's cities.
Liz Truss will join fellow foreign ministers from Nato and the European Union for a series of meetings in Brussels as the allies show their support for Ukraine.
Meanwhile Home Secretary Priti Patel is visiting Poland's border with Ukraine to highlight the visas on offer to those fleeing the conflict who have relations in Britain.
For the first time since Brexit, Foreign Secretary Ms Truss will attend the EU's Foreign Affairs Council, as a special guest along with counterparts from the US, Canada and Ukraine.
She will also attend a special meeting of Nato foreign ministers and hold talks with counterparts from the G7 group.
Ms Truss said it was "one of the biggest days of diplomacy" with allies prepared to "tighten the vice around Putin's war machine" by targeting the Russian economy with sanctions.
The West fears the Russian president will unleash an overwhelming assault on Ukraine's major cities, potentially inflicting devastating civilian casualties.
French President Emmanuel Macron spoke to Mr Putin on Thursday but said "he refuses to stop his attacks on Ukraine at this point".
The Russian president warned Ukraine that it must quickly accept the Kremlin's demand for its "demilitarisation" and declare itself neutral, renouncing its bid to join Nato.
Peace talks between the two sides have so far failed to end the hostilities, although there is hope for humanitarian corridors to allow civilians to escape.
The port city of Kherson became the first major city to fall since the invasion began.
Russian forces continued pressing on multiple fronts, but a long column of tanks has apparently been stalled outside the capital Kyiv for days.
At least 33 civilians were killed and 18 wounded in a strike on a residential area in the northern city of Chernihiv, Ukraine said.
Mr Putin's forces have fired more than 480 missiles in the invasion, according to a US defence official.
Western allies have responded to the ongoing violence with increased sanctions.
Russian billionaire Alisher Usmanov, who has had ties to Arsenal and Everton football clubs, was sanctioned by the UK and US.
A travel ban and full asset freeze was also imposed on former Russian deputy prime minister Igor Shuvalov on Thursday evening, taking the total number of oligarchs sanctioned by the UK to 15.
Boris Johnson said: "For as long as Putin continues his barbaric attack on innocent Ukrainians we will continue to exert every power we have to inflict maximum economic pain on Putin and his war machine."
But Roman Abramovich, who says he will sell Chelsea FC, was not among the latest tranche of sanctioned oligarchs.
Government sources conceded it could take "weeks and months" to build legally sound cases against wealthy and litigious targets.
Meanwhile the exodus from Ukraine continued, with UN refugee agency figures showing more than a million people have fled the country, a figure which could rise to four million as the war rages on.
The Home Secretary will travel to Medyka on Poland's border with Ukraine to see the situation for herself.
Ahead of the visit she said: "The British Government will do everything it can to support the Ukrainian people at this critical moment as they fight for freedom."
The Ukraine family scheme will allow Britons or people settled in the UK to bring their relatives to safety in Britain.
But the Government has faced calls to go further in opening up routes for Ukrainians to come to the UK.
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