The leaders of 26 countries have agreed to move to an “operational phase” in planning for a multinational force to guarantee a ceasefire in Ukraine, UK prime minister Sir Keir Starmer has said, adding: “Let the guns fall silent.”

Starmer said military planners would meet in London on Thursday to discuss how a truce could be guaranteed “by strong security arrangements through our coalition of the willing”.

After hosting a video call of the 26 mainly European leaders on Saturday, Starmer admitted the US was still not prepared to provide a military “backstop” to ensure any peace was durable.

“The Kremlin’s dithering and delay over President Trump’s ceasefire proposal, and Russia’s continued barbaric attacks on Ukraine, run entirely counter to President Putin’s stated desire for peace,” Starmer said. “A position of ‘Yes, but . . . ’ isn’t good enough.”

Speaking in Downing Street, Starmer confirmed that the group of countries would “double down and increase sanctions” on Russia, including looking at the option of seizing Russian assets to intensify pressure on Putin.

British officials said a number of countries had made new offers to send troops to Ukraine to help secure a peace deal but declined to say which ones.

“The position of the US has not changed,” Starmer said, referring to the idea of American military support for a UK-France led peace guarantee force. “We have been clear this has to be done in conjunction with the US.” He said Britain was speaking to the US on a daily basis.

Starmer launched what he called a coalition of the willing at a summit in London two weeks ago, with the assembled countries considering varying levels of support for Ukraine.

Most of the countries on Saturday’s call were from Europe; the participants also included Australia, Canada and New Zealand. The EU and Nato were represented along with Ukraine. Starmer said that Japan, which was not on the call, had offered “backing”.

Britain and France are the only countries to have firmly committed to send troops to Ukraine to help guarantee a peace deal, although others are open to the idea. Some have offered financial or logistical support.

The leaders agreed to make preparations for a force to be sent to Ukraine in the event of a peace agreement to act as a “robust” guarantee, with talks at a military planner level due to be held in London on Thursday.

“We agreed to accelerate our practical work to support a potential deal. So we will now move into an operational phase,” the British prime minister said.

Starmer said the leaders had agreed to increase financial and military support for Ukraine in the short term but also in the longer term, to help Kyiv provide its own security guarantees.

He stressed the need for US backing for any peacekeeping operation, saying the risk was that “we will repeat the experience of recent years and Russia will breach the line again”.

The prime minister also confirmed that the leaders discussed beefing up sanctions if Putin did not agree to a ceasefire. Downing Street said that seizing more than €200bn of frozen Russia assets was being considered.

Number 10 conceded that seizing Russian assets — as opposed to using interest accruing from them to support Ukraine, as it currently does — was “a complex situation”.

Starmer said that the leaders would “continue to explore all lawful routes to ensure that Russia pays for the damage it has done to Ukraine”.

France and Germany, which have long opposed a full-blown seizure of assets held in the EU, are warming to the idea and are discussing with the UK others ways in which they could be used as part of a negotiation to end the war.

While Britain and France have offered to put troops on the ground to help secure the peace — but only if a proposed initial 30-day ceasefire holds — other countries have been more reluctant to commit direct military support.

Starmer told the meeting that the “coalition of the willing” had to help “monitor a ceasefire” during the truce. That would initially involve satellites and surveillance rather than the deployment of troops to Ukraine, UK officials said.

Countries including Germany and Poland have reservations about sending troops to Ukraine. The office of Giorgia Meloni, Italy’s prime minister, said Italian participation in a “possible military force on the ground was not envisaged”.

Dick Schoof, Dutch prime minister, said on X that the Netherlands would join the meeting of military planners on Thursday “in the context of security guarantees”.

Ursula von der Leyen, European Commission president, said the ball was in Putin’s court. “In the meantime, we will support strengthening Ukraine and its armed forces,” she said.

“We will step up Europe’s defence efforts through ReArm Europe,” she added, referring to new Brussels initiatives aimed at increasing national defence spending.

German chancellor-to-be Friedrich Merz, whose Christian Democratic Party won elections last month, has reached a deal with the country’s mainstream parties to inject hundreds of billions into Germany’s defence sector and exempt military aid for Ukraine from strict borrowing limits. The German parliament is due to vote on the constitutional changes on Tuesday.

Ukrainian President Volodymyr Zelenskyy urged European leaders to “massively” increase military production.

“Everyone understands that at the current pace, Ukraine and Europe can be strengthened within five years. We believe this is too long and too dangerous,” Zelenskyy said on X after the online summit. “If the Russians do not agree, it means they are not disagreeing with our proposal, but specifically with President Trump’s proposal.”

Additional reporting by Amy Kazmin in Rome, Anne-Sylvaine Chassany in Berlin and Andy Bounds, Henry Foy in Brussels and Fabrice Deprez in Kyiv