British military personnel are to be sent to Ukraine in the next few weeks to provide advice and training, says Prime Minister David Cameron.
As he spoke, pro-Russian rebels said they were pulling back heavy weapons from the front line in eastern Ukraine as part of a ceasefire.
However, the government in Kiev has disputed the rebel claims.
Mr Cameron said he would push for more sanctions on Russia if the separatists failed to observe the truce.
Foreign ministers from Russia, Ukraine, Germany and France met in Paris on Tuesday in an attempt to bolster confidence in the ceasefire agreed in Minsk on 12 February but made little progress.
Ukraine fears that the separatists are planning to use the relative lull in fighting to switch their attention to the southern port city of Mariupol.
The United States has already said it is planning to send troops to train three Ukrainian battalions.
Mr Cameron told a House of Commons committee that up to 75 British soldiers divided into four teams would go to an area well away from the conflict zone, to provide medical, logistics, intelligence and infantry skills.