"On the coalition of the willing, yes, a number of countries indicated today that they want to be part of the plan that we are developing," Starmer told a news conference following a meeting of European leaders in London.
"I'll leave them to make their own statements about exactly how they want to make that contribution but we've been able to move that forward," he said.
"I accept, without criticism and with respect, the position of other countries that may not feel that they want to contribute in that way but I strongly feel that unless some move forward, we will stay in the position we're in and not be able to move forward."
Starmer urged European leaders to join the so-called "coalition of the willing" alongside France and the UK, aimed at keeping the peace if the Ukraine war is brought to a negotiated end.
The UK prime minister said the leaders had agreed to "meet again very soon" to keep up work on the plans.
"We are at a crossroads in history today," he said.
"This is not a moment for more talk. It's time to act. Time to step up and lead, and to unite, around a new plan for a just and enduring peace."
Starmer announced a 1.6 billion pound ($A3.2 billion)deal which would allow Ukraine to purchase 5000 air-defencemissiles using export finance, following the gathering at Lancaster House in central London.
NATO Secretary General Mark Rutte said that some European leaders had privately set out new plans on military spending during the London meeting but declined to give details.
"Today at the table I heard new announcements, and I'm not going to announce them to you, because they should do this. But this was very good news that more European countries will ramp up defence spending," Rutte told reporters outside the meeting on Sunday.
Starmer urged European leaders on Sunday to step up their defence efforts to not only help secure peace in Ukraine but stability across the continent at a summit staged as a show of support for Ukrainian President Volodymyr Zelenskiy.
Just two days after a contentious Oval Office exchange between US President Donald Trump and Zelenskiy, the Ukrainian leader was welcomed in London by world leaders, who hugged him in turn, and pledged their backing.
Starmer said Europe needed to meet a once-in-a-generation challenge.
"Getting a good outcome for Ukraine is not just a matter of right and wrong, it is vital for the security of every nation here and many others, too," Starmer said at the beginning of the meeting flanked by Zelenskiy and French President Emmanuel Macron.
"Today's meeting is to unite to discuss how to deliver a just and enduring peace together and to make sure Ukraine can defend and protect against any future Russian attack."
Trying to revive hope for peace in Ukraine, Starmer said urgent talks with Trump, Zelenskiy and Macron over the weekend had cemented the idea that a "coalition of the willing" in Europe would need to move quickly to come up with a peace plan to be presented to the United States.
Polish Prime Minister Donald Tusk, who also attended the summit, said "Europe has woken up".
After the meeting ended, he wrote on X in Polish that the European Union, the United Kingdom, Norway and Turkey now "all speak with one voice about aiding Ukraine, the need for close transatlantic co-operation, and strengthening the eastern border".
European Commission President Ursula von der Leyen said she will present next week a plan to "rearm Europe" to bolster the European Union's security.
She said she will unveil the plan to heads of state and government during an EU summit in Brussels.
"We all have understood that after a long time of under-investment it is now of utmost importance to step up the defence investment for a prolonged period of time. It's for the security of the European Union," she said.
Von der Leyen added that the plan could for instance help develop advanced air shields.
with AP and PA
