The UK and France have agreed in principle to deploy troops to Ukraine if a peace agreement with Russia is reached.
UK Prime Minister Sir Keir Starmer said a declaration of intent was signed following talks with allies in Paris.
He said the plan includes establishing military hubs across Ukraine to deter future Russian aggression.
French President Emmanuel Macron said the deployment could involve thousands of troops.
Allies broadly agreed on the need for strong security guarantees for Ukraine after a ceasefire.
The United States is expected to lead monitoring efforts to verify any truce.
Russia has warned that foreign troops in Ukraine would be considered legitimate military targets.
Moscow currently controls around 20% of Ukrainian territory following its 2022 invasion.
We signed a declaration of intent on the deployment of forces to Ukraine in the event of a peace deal.
Sir Keir Starmer said.
He added the agreement would create a legal framework for British, French and partner forces to operate in Ukraine.
US negotiator Steve Witkoff said durable security guarantees were essential for a lasting peace.