By Andrew Gray, Lili Bayer and John Irish

BRUSSELS (Reuters) - European leaders warned the United States on Thursday against sealing a Ukraine peace deal with Russia behind their backs as they scrambled for a seat at the table after Donald Trump spoke to Vladimir Putin and announced the start of negotiations.

U.S. President Trump's move sent shockwaves through European capitals, which want a central role in peace talks because any settlement in Ukraine, hit by a full-scale Russian invasion three years ago, will have ramifications for their own security.

"It is clear that any deal behind our backs will not work. Any agreement will need also Ukraine and Europe being part of it," European Union foreign policy chief Kaja Kallas said.

"Why are we giving them (Russia) everything that they want even before the negotiations have been started?" said Kallas, speaking before a meeting of NATO defence ministers with their Ukrainian counterpart in Brussels.

"It's appeasement. It has never worked."

U.S. Defense Secretary Pete Hegseth declared on Wednesday it was unrealistic for Ukraine to return to pre-2014 borders and that NATO membership for Kyiv would not be part of a peace deal. Russia seized and annexed Ukraine's Crimea peninsula in 2014.

German Defence Minister Boris Pistorius also said it would have been better if Washington had not given what he called concessions to Moscow before peace talks even started.

Kallas said any agreement without European and Ukrainian participation would fail and Europe would continue to support Kyiv if it resisted a settlement that was imposed upon it.

"Any quick fix is a dirty deal," said Kallas.

French Defence Minister Sebastien Lecornu warned against "peace through weakness" rather than the Trump officials' mantra of "peace through strength".

Lithuanian Defence Minister Dovile Sakaliene said Europe should not fall "under the illusion that Mr. Trump and Mr. Putin are going to find the solution for all of us".

Defending the U.S. approach, Hegseth said the world was fortunate to have Trump, the "best negotiator on the planet, bringing two sides together to find a negotiated peace".

EUROPEAN HOPES

Many European officials had hoped that a series of meetings with officials in Brussels and Munich this week would be their opportunity to influence U.S. thinking on the war.

But it soon became clear that the Trump administration was moving ahead without them.

Hours after Hegseth spoke, Trump said he had held a "highly productive phone call" with Russia's Putin and they had agreed to start negotiations immediately. He then briefed Ukrainian President Volodymyr Zelenskiy on the call.

In a late-night statement after talks in Paris on Wednesday, foreign ministers from European powers - including Britain, France and Germany - said Europe must be part of any future negotiations on Ukraine.

A European diplomatic source said the ministers agreed to engage in a "frank and demanding dialogue" with U.S. officials at the annual Munich Security Conference - a three-day gathering that starts in the southern German city this Friday.

Not all European leaders were alarmed by Trump's move.

Hungarian Prime Minister Viktor Orban, a Trump ally, praised the U.S. president and criticised the ministers' statement.

"You can't request a seat at the negotiating table. You have to earn it!" he said on social media.

European leaders say one reason they need to be involved in talks is that Washington has made clear it expects them to provide security guarantees for any peace deal, which could mean European troops being deployed to Ukraine.

"There is no option to not be at the table, because we are very important in the actual implementation of those security guarantees," said Dutch Defence Minister Ruben Brekelmans.

(Additional reporting by Bart H. Meijer and Ludwig Burger; editing by Mark Heinrich and Gareth Jones)