Addressing a virtual meeting of the European leaders following the call with Trump, Sir Keir said “any ceasefire would have to be lasting and to be lasting it would need security guarantees”.
“That is why we set up this coalition of the willing,” he added.
The coalition is a group of mainly European countries who have pledged to provide military support to Ukraine – including potentially boots on the ground – in order to deter Russia from breaching any agreed peace deal.
Sir Keir said the coalition had “credible” military plans ready that could be used in the event of a ceasefire.
He said the leaders of the group were also ready to increase economic pressure on Russia if necessary, for example through increasing sanctions.
He also praised Trump’s efforts to reach an agreement, saying: “For three-and-a-bit years this conflict has been going on and we haven’t got anywhere near the prospect of an actually a viable solution, a viable way of bringing it to a ceasefire.
“Now we do have that chance, because of the work the president has put in.”