The meeting was the third of its kind since Starmer earlier this month announced he would work to bring together nations who could guarantee any cessation of hostilities between Russia and Ukraine.

"Europe's two leading nuclear powers are stepping up to forge a stronger and deeper partnership that has a vital foundational role to play in the security of the continent," said Chief of the Defence Staff Admiral Tony Radakin afterwards.

"Today's deliberations will help shape the future of British and French military co-operation, and our shared efforts to support Ukraine with military aid now and after any peace deal," he added.

After the meeting, the defence chiefs met King Charles III -- head of the British armed forces -- at Windsor Castle, west of London.

Starmer's Downing Street office said it was continuing to keep the US updated on military planning meetings.

"The coalition of the willing is a group of nations politically aligned to the defence, security and sovereignty of Ukraine ... if there is a deal, it's a deal that has to be defended," his official spokesman said.

Starmer has been leading efforts to form the group alongside French President Emmanuel Macron since US President Donald Trump opened direct negotiations with Russia last month to end the three-year war.

But questions remain over what the group might be able to do after Putin demanded an end to Western military aid to Ukraine as a condition for any end to fighting.

Starmer and Macron have said they are willing to put British and French troops on the ground in Ukraine, but Russia has said that it would not agree to soldiers from NATO countries being deployed there.

The prime minister has said he welcomes any offer of support for the group, raising the prospect that some countries could contribute logistics or surveillance.

Monday's meeting was attended by the heads of the British and French armies, navies and air forces as well as the head of both countries' defence forces.