KYIV, Ukraine — Senior military officers from more than two dozen countries across Europe and beyond met Thursday in England to flesh out plans for an international peacekeeping force for Ukraine as details of a partial ceasefire are worked out.

U.K. Prime Minister Keir Starmer said he didn’t know whether there would be a peace deal in the Russia-Ukraine war, but “we are making steps in the right direction” as a “coalition of the willing” led by Britain and France moves into an “operational phase.”

“We hope there will be a deal but what I do know is if there is a deal, the time for planning is now,” he said during a visit to the meeting of military planners at a British base in Northwood, just outside London. “It’s not after a deal is reached.

“It is vitally important we do that work, because we know one thing for certain, which is a deal without anything behind it is something that (Russian President Vladimir) Putin will breach,” he said.

Ukraine and Russia agreed in principle Wednesday to a limited ceasefire after President Donald Trump spoke with the countries’ leaders this week, though when it might take effect and what possible targets would be off limits remain to be seen.

Ukrainian President Volodymyr Zelenskyy, speaking Thursday in Norway, said that although he originally had sought a broader ceasefire, he was committed to working with the U.S. to stop arms being directed at power production and civilian facilities.

“I raised this issue with President Trump and said that our side would identify what we consider to be civilian infrastructure,” Zelenskyy said. “I don’t want there to be any misunderstanding about what the sides are agreeing on.”

The tentative deal to partially rein in the three-year war came after Putin rebuffed Trump’s push for a full 30-day ceasefire. The difficulty in getting the combatants to agree not to target one another’s energy infrastructure highlights the challenges Trump will face in trying to fulfill his campaign pledge to quickly end to the war.

Negotiators from Moscow and the U.S. will meet Monday in Riyadh, Saudi Arabia, Putin’s foreign affairs adviser Sergei Ushakov told Russian news agencies.

Zelenskyy said his team would also meet with the U.S. in Saudi Arabia to discuss technical issues, and then the U.S. will act as an intermediary running “shuttle diplomacy” between Kyiv and Moscow.

Despite the negotiations, hundreds of drone attacks were launched overnight by both sides, injuring several people and damaging buildings.

Kropyvnytskyi, a city in central Ukraine, faced its biggest attack of the war as about four dozen drones injured 14 people, including a couple with serious burns, and damaged houses and apartments.

“In a cruel twist, enemy drones hit Myru Street (‘Peace Street’ in English),” said Andrii Raikovych, head of the regional administration.

More than 50 drones were intercepted in Russia’s Saratov region — the largest attack of its kind in the area — shattering windows in a hospital and damaging two kindergartens, a school and about 30 homes, Gov. Roman Busargin said. The attacks were focused on Engels, an industrial city near Russia’s main base for nuclear-capable strategic bombers.

In its latest estimate, the U.K. Defense Ministry said Russian troops suffered 900,000 casualties — including up to 250,000 killed — since Moscow’s invasion of Ukraine three years ago. That’s a jump of 200,000 from a fall estimate.

Western estimates of the parties’ war losses have varied and couldn’t be independently verified.

War losses have been a tightly guarded secret in Russia.

The U.K. did not release a similar estimate for Ukrainian casualties.

Zelenskyy told NBC News last month that more than 46,000 Ukrainian soldiers had been killed, and more than 350,000 wounded. Those figures couldn’t be independently confirmed and could be an undercount.

If peace comes to Ukraine, the number of troops that would help enforce it is vague. Officials have cited figures of 10,000 to 30,000 troops as part of what has been called a “reassurance force.”

Only Britain and France have said they are willing to send troops, though countries including Australia, Canada, France and Finland say they are open to being involved in some way.

Russia has said it will not accept any troops from NATO countries being based on Ukrainian soil. And Trump has given no sign the U.S. will guarantee reserve firepower in case of any breaches of a truce.

Starmer says the plan won’t work without that U.S. “backstop.”