Reported by BBC World
A British paratrooper takes part in a previous Nato training exercise less than 50km (31 miles) from the Russian border in Finland
Britain and France are to lead a military exercise to test a multinational force's ability to deploy to Ukraine, if there is a peace deal.
Nearly 3,000 military personnel from 12 European nations will take part in the rehearsal in neighbouring Poland this month.
More from the original report
Plans for a reassurance force for Ukraine were set out last year as one of the long-term security guarantees being proposed by European nations.
But there are still more questions than answers as to how such a military force might work in practice. Western officials stress that any force sent to Ukraine would not take part in offensive operations or be involved in peacekeeping or ceasefire monitoring missions.
Nor would they be operating under the command of Nato or on the front line.
Officials say their presence would be to provide reassurance to Ukraine and to help rebuild its armed forces.
The US has been involved in discussions, but is not contributing to the mission.
Russia has already voiced opposition to foreign troops being stationed in Ukraine and, last year, President Vladimir Putin said they would be "legitimate targets".
Western officials have declined to spell out the rules of engagement if any international troops were to be attacked in Ukraine. Though they say the force would be able to defend itself.
UK Defence Secretary Wes Streeting said the exercise showed Britain, France and their allies were stepping up preparation for the day when peace came to Ukraine.
He said: "This is not a hypothetical plan, it's a real force, ready to help Ukraine once the fighting stops."
Lt Gen Tom Bateman, the UK commander of the Multi National Force – Ukraine (MNF-U), said the exercise represented an important step in translating political commitments into credible military readiness.
He said it was a "real, capable, readily deployable force that represented a serious commitment from Coalition of the Willing nations to Ukraine's long-term stability and security".
Ukrainian troops take part in urban warfare training at a…