Denouncing Russia's "barbaric strikes" on Ukraine, the UK is "stepping up" by "choking off the revenues that fuel Putin's war and powering Ukraine through the winters ahead", his office quoted Starmer as saying, referring to Russian President Vladimir Putin.

Ukrainian President Volodymyr Zelensky was due to attend the summit of seven leading world powers in the French resort town of Evian-les-Bains on Tuesday, with allies looking to tilt the balance in Kyiv's favour after over four years of war.

His appearance comes after a Russian barrage of missiles early Monday killed at least 11 people across Ukraine and sparked a fire at one of the most important Orthodox monasteries in the capital.

Starmer's office said the energy deal would "power Ukraine for the next two years".

The British premier -- under pressure at home after his defence minister resigned in a row over military spending last week -- will tell leaders at the session that "the G7 should collectively go further to ensure Ukraine secures the just and lasting peace it deserves".

"We will stand with Ukraine for as long as it takes and this announcement reinforces that," Starmer said, according to his office.

Some pound210 million ($282 million) worth of export finance will allow the UK-based Urenco to supply enriched uranium to Ukraine's nuclear power producer, Energoatom, it said.

The new sanctions "will choke Russia's war effort across multiple fronts" by targeting Russia's illicit shadow fleet and finance networks used to circumvent sanctions, Starmer's office said.