But many questions remain unanswered, from the size of any force, to who would contribute, what the mandate would be and whether the United States would back it up.

Here's what we know about the planning so far:

- 'Reassurance' not 'peacekeeping' force -

London and Paris describe the possible deployment as a "reassurance force" aimed at offering Ukraine some kind of security guarantee in the event Trump strikes a deal.

No one is saying yet where in the war-torn country any foreign troops could be based -- but one thing seems clear, they wouldn't be on the front line.

"This is not a peacekeeping force that will separate the currently warring sides down the line of contact," Britain's defence minister John Healey said after talks in Brussels on Thursday.

Healey said the deployment had several objectives, "first to secure safe skies, second, to secure safe seas, third, to support a peace on the land, and fourth, to support the Ukrainian armed forces to become their own strongest possible deterrent."

The UK media reported in February that initial discussions had put the size of any force at fewer than 30,000 troops -- but diplomats have since said it would likely be smaller.

- Rebuild Ukraine's forces -

Officials increasingly stress that a key purpose for any foreign deployment would be to give Ukraine's forces time to reconstitute after over three years of gruelling war.

"Ultimately, these security guarantees are also there to allow for a path of regeneration for the Ukrainian armed forces," French defence minister Sebastien Lecornu said.

That could mean Western forces would be involved in training their Ukrainian counterparts.

Western intelligence agencies estimate that if peace comes in Ukraine it would take Russia three to five years to fully rebuild its forces -- giving that timeframe to help Kyiv get up to strength.

Diplomats say that once Ukraine's military is strong enough, the foreign force could be scaled back.

- Rules of engagement -

For now all the planning remains hypothetical.

Trump's efforts to agree a ceasefire have stalled and until an agreement is in place it is near impossible to set precise rules for any force.

"There are difficult questions that are not answered yet: what exactly our mandate would be, what we would do there when Russia violates the ceasefire," said Lithuania's defence minister Dovile Sakaliene.

So far only a handful of countries -- including Britain, France and the Baltic states -- have said they'd be willing to send troops to Ukraine.

Russia has said it would not agree to troops from any NATO countries deploying in Ukraine.

- US role -

One major question is if the United States could have any role in backing up the force.

Washington has so far suggested it would be down to the Europeans to do the heavy lifting.

But multiple European countries insist they'd want to see the United States involved somehow -- either offering air cover and logistical support from a distance, or at least being ready to step in if the situation escalates.

"It's important to keep the United States on board," said Dutch defence minister Ruben Brekelmans.

"You can have different types of missions with different risk profiles, and in general, the higher the risk of confrontation then the more important it is the United States is closely involved."