Ukraine army to be trained by UK troops - Cameron
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.
The prime minister 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.
Analysis: Jonathan Marcus, BBC Defence Correspondent
Over the past year small numbers of British troops have already taken part in Nato exercises in Ukraine, but this is the first explicit training mission there.
The US is to start its own training mission in a few weeks time and where the US and Britain are leading, other Nato countries are likely to follow. The point of all this is to send a clear signal to Moscow of the West's determination to stand by the Kiev government.
While much of the British training will be in non-combat or combat-support areas like medical skills; logistics and intelligence, there will also be an infantry element to the programme which the Ministry of Defence says will include things like the planning and execution of urban operations.
Mr Cameron made it clear that, for now, there was no question of selling weapons to Ukraine, but with the ceasefire failing, the debate is likely to be renewed in Washington with vigour.
In a statement, British Defence Secretary Michael Fallon said that the move was a result of "continued Russian-backed aggression".
The United States has already said it is planning to send a battalion of soldiers to train three Ukrainian battalions.
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. They made little progress, reports said.
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.
Following talks in Paris, Ukrainian Foreign Minister Pavlo Klimkin told reporters: "We are extremely concerned by recent attacks in the vicinity of Mariupol. We are concerned by a possible relocation of forces from Debaltseve in the direction of Mariupol."
At the scene: Paul Adams, BBC News, east Ukraine
On the road about half an hour's drive east of Donetsk, a convoy of trucks towed large-calibre artillery pieces heads away from the front lines. Evidence, the rebels said, of a willingness to withdraw heavy weapons.
After posing for the cameras, the convoy moved eastwards, but no-one told us where it was going.
We would have loved to follow that convoy a little further, but there was a man in a jeep right in front of us, and every time we tried to overtake, he would not let us. He waved his gun in the air, which I took as a way of telling us to back off.
We saw two monitors from the Organization for Cooperation and Security in Europe (OSCE), but later learned it was quite by chance that the monitors saw the convoy.
And that is worrying. A week after the OSCE wrote to both sides, asking for detailed plans of what they intended to do with their heavy weapons, it has yet to hear back.
International monitors say they cannot confirm the withdrawal of the rebel weaponry in the east, though movement of artillery columns has been reported.
Monitors at the Organization for Security and Co-operation in Europe say it is not possible to verify the pullout as they do not know the amount of weapons removed or where they have been taken.
Meanwhile, US Secretary of State John Kerry accused Russia of repeatedly lying about their involvement in Ukraine.
"And they have been persisting in their misrepresentations [or] lies, whatever you want to call them, about their activities there to my face," Mr Kerry told a Senate Committee.
Almost 5,800 people have died since the fighting in Ukraine erupted last April, the UN has estimated, although the organisation believes the real figure could be considerably higher.