UK Prime Minister Keir Starmer said Russian President Vladimir Putin's "promises are hollow" on any eventual ceasefire with Ukraine, as Kyiv's allies met in Paris for a summit.

"Unlike (Ukrainian) President (Vladimir) Zelensky, Putin has shown he's not a serious player in these peace talks," Starmer said.

"Playing games with the agreed naval ceasefire in the Black Sea despite good faith participation from all sides -- all while continuing to inflict devastating attacks on the Ukrainian people. His promises are hollow," he said.

The British leader said the United States "is playing a leading role by convening the ceasefire talks", adding, "President Zelensky has demonstrated his commitment repeatedly, and Europe is stepping up to play its part to defend Ukraine's future.

"Now Putin needs to show he's willing to play ball," Starmer added.

French President Emmanuel Macron was hosting European leaders including President Volodymyr Zelensky for a summit aimed at boosting Ukrainian security ahead of any potential ceasefire with Russia to end the over three-year war.

The meeting that began later yesterday was seeking to set out what security guarantees Europe can offer Ukraine once a ceasefire deal is agreed, including the possible deployment of military forces by a so-called "coalition of the willing".

Macron said that France and Britain were leading efforts to send a "reassurance force" to Ukraine after any peace in the conflict with Russia, even if the idea was not approved by all European allies.