Ukrainian President Volodymyr Zelenskiy thanked European leaders on Sunday for backing his demand for a seat at the table as Russia and the United States prepare for a summit this week where Kyiv fears they could seek to dictate terms to it for ending the 3-1/2-year war.

US President Donald Trump, who for weeks had been threatening new sanctions against Russia for failing to halt the conflict, announced instead last Friday that he would hold an August 15 summit with Russian President Vladimir Putin in Alaska.

A White House official said on Saturday that Trump was open to Zelenskiy attending, but that preparations currently were for a bilateral meeting with Putin.

The Kremlin leader last week ruled out meeting Zelenskiy at this point, saying the conditions for such an encounter were "unfortunately still far" from being met.

Trump said a potential deal would involve "some swapping of territories to the betterment of both (sides)", a statement that compounded Ukrainian alarm that it may face pressure to surrender more land.

Zelenskiy says any decisions taken without Ukraine will be "stillborn" and unworkable. On Saturday the leaders of Britain, France, Germany, Italy, Poland, Finland and the European Commission said in a joint statement that any diplomatic solution must protect the vital security interests of Ukraine and Europe.

"The path to peace cannot be decided without Ukraine," they said, demanding "robust and credible security guarantees" to allow Ukraine to defend its sovereignty and territorial integrity.

Zelenskiy said on Sunday: "The end of the war must be fair, and I am grateful to everyone who stands with Ukraine and our people today for the sake of peace in Ukraine, which is defending the vital security interests of our European nations."

A European official said Europe had come up with a counter-proposal to Trump's, but declined to provide details. Russian officials accused Europe of trying to thwart Trump's efforts to end the war.

"The Euro-imbeciles are trying to prevent American efforts to help resolve the Ukrainian conflict," former Russian president Dmitry Medvedev posted on social media on Sunday.

Russian Foreign Ministry spokeswoman Maria Zakharova said in a vituperative statement that the relationship between Ukraine and the European Union resembled "necrophilia".

Roman Alekhin, a Russian war blogger, said Europe had been reduced to the role of a spectator.

"If Putin and Trump reach an agreement directly, Europe will be faced with a fait accompli. Kyiv - even more so," he said.

CAPTURED TERRITORY

No details of the proposed territorial swap that Trump alluded to have been officially announced.

Russia, which mounted a full-scale invasion of Ukraine in February 2022, holds about a fifth of the country and has claimed the regions of Luhansk, Donetsk, Kherson and Zaporizhzhia as its own, although it controls only about 70% of the last three.

Russia has also taken pockets of territory in the Sumy and Kharkiv regions and said in recent weeks it has captured villages in the Dnipropetrovsk region. Ukraine says it holds a sliver of the Kursk region in western Russia.

Sergei Markov, a pro-Kremlin analyst, said a swap could entail Russia handing over 1,500 sq km to Ukraine and obtaining 7,000 sq km, which he said Russia would capture anyway within about six months.

He provided no evidence to back any of those figures. Russia took only about 500 sq km of territory in July, according to Western military analysts who say its grinding advances have come at the cost of very high casualties.

Ukraine and its European allies have been haunted for months by the fear that Trump, keen to claim credit for making peace and hoping to seal lucrative joint business deals between the U.S. and Russia, could align with Putin to cut a deal that would be deeply disadvantageous to Kyiv.

They had drawn some encouragement lately as Trump, having piled heavy pressure on Zelenskiy and berated him publicly in the Oval Office in February, began criticising Putin and expressing disgust as Russia pounded Kyiv and other cities with its heaviest air attacks of the war.

But the impending Putin-Trump summit, agreed during a trip to Moscow by Trump's envoy Steve Witkoff last week, has revived fears that Kyiv and Europe could be sidelined.

"What we will see emerge from Alaska will almost certainly be a catastrophe for Ukraine and Europe," wrote Phillips P. O'Brien, professor of strategic studies at the University of St Andrews in Scotland.

"And Ukraine will face the most terrible dilemma. Do they accept this humiliating and destructive deal? Or do they go it alone, unsure of the backing of European states?"