Starmer said military chiefs would meet in London on Thursday to "put strong and robust plans in place to swing in behind a peace deal and guarantee Ukraine's future security".
His comments came after a virtual meeting with 29 other world leaders, including French President Emmanuel Macron, Australian Prime Minister Anthony Albanese and Ukrainian President Volodymyr Zelenskiy.
Starmer said new commitments had been made on peacekeeping and tightening sanctions on Russia.
"There is a stronger collective resolve and new commitments were put on the table," he told reporters.
"We now move to the operational phase."
He did not elaborate on which nations had made concrete commitments to a peacekeeping force.
Asked about what action a peacekeeping force could take, he said this would be a subject of discussion between military chiefs on Thursday.
"There are different capabilities from different countries, but those are the operational discussions that are going on in relation to what this coalition of the willing will be able to provide," he said.
Among the other attendees at Saturday's meeting were Italian Prime Minister Giorgia Meloni, who had been reportedly considering not joining the call due to scepticism about Anglo-French peacekeeping proposals.
As well as European nations, the leaders Canada and New Zealand also joined the call, as did NATO secretary-general Mark Rutte.
Saturday's meeting followed an intense week of diplomacy in which American officials put a US-Ukraine proposal for an unconditional 30-day ceasefire to Russia.
But the Kremlin has so far resisted the proposal, saying it would only agree to a ceasefire if Ukraine also agreed to abandon its aim of joining NATO and gave up some of its territory to Russia.
Speaking from the cabinet room in Number 10 on Saturday, Starmer told world leaders they could not "sit back and wait" for a ceasefire in Ukraine.
"We have to keep pushing ahead, pushing forward and preparing for peace, and a peace that will be secure and that will last," he said.
Starmer also accused Vladimir Putin of trying to delay a ceasefire, adding that "sooner or later he is going to have to come to the table" and engage in serious discussions.
Earlier on Saturday, Mr Putin's first prime minister Mikhail Kasyanov, who now opposes the Russian president, said his former boss would not accept NATO or European troops being deployed to Ukraine but might agree to soldiers from "friendly countries" such as India and Brazil.
Following the meeting, Liberal Democrat leader Ed Davey urged the UK to seize Russian assets and use them to fund more support for Ukraine, saying this was "the only way to achieve a just and lasting peace".
Asked about whether he had discussed seizing Russian assets with his counterparts, Starmer said it had been discussed, but added it was "a complicated question".
