US President Donald Trump, who for weeks had been threatening new sanctions against Russia for failing to halt the conflict, has announced he will hold a summit with Russian President Vladimir Putin in Alaska on Friday.
A White House official said on Saturday Trump was open to Zelenskiy attending, but that preparations 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 might 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.
"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," Zelenskiy said on Sunday.
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.
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, although it controls only about 70 per cent of the last three.
Russia has also taken pockets of territory in the Sumy and Kharkiv regions and said in recent weeks it had captured villages in the Dnipropetrovsk region.
Ukraine says it holds a sliver of the Kursk region in western Russia.
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 US 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 Kyiv and Europe could be sidelined.
