They could not comment on the final make-up and shape of a force that might be deployed if the fighting stops. Commitment and contributions would depend on the circumstances of a halt in hostilities and on individual nations’ political decisions.
In the past, military experts have suggested Western nations might have to send tens of thousands of troops to Ukraine to guarantee its security. Questions remain about how such a force would work in practice.
It is also unclear what training role Western forces could provide to battle-hardened Ukrainian troops. Ukrainian soldiers are already in high demand training Nato forces, passing on expertise in drone warfare.
Western officials stress that any force sent to Ukraine would not take part in offensive operations. They also say it would not be involved in peacekeeping or ceasefire monitoring.
Officials say the presence would be to provide reassurance to Ukraine and to help rebuild its armed forces. They add that the force would be able to defend itself.
Plans for a reassurance force for Ukraine were set out last year as one of the long-term security guarantees proposed by European nations.
Western officials have declined to spell out the rules of engagement if any international troops were attacked in Ukraine.
The exercise will also test command and control so that troops from different countries can work together.
Nearly 3,000 military personnel from 12 European nations will take part in the rehearsal in neighbouring Poland, across land, sea and air.
A British official said the plan was not hypothetical. “This is not a hypothetical plan, it’s a real force, ready to help Ukraine once the fighting stops.”.