US President Donald Trump is pressing Ukraine to accept his 28-point proposal by Thursday.
The terms of the deal mirror many of Russia's sweeping demands, including the surrender of Ukraine's entire eastern Donbass region and a cap on the size of its armed forces.
The plan has sparked alarm in Kiev and across European capitals, prompting a crisis meeting of European leaders on the sidelines of the G20 summit in Johannesburg, South Africa on Saturday.
Details of the changes presented to the Trump administration have not been disclosed but European representatives at the meeting in the Swiss city on Sunday are expected to push back against what they see as a peace deal heavily tilted in Moscow's favour.
Ukrainian and EU representatives are also due to take part in the meeting, which will be held at adviser level.
European leaders, plus the prime ministers of Japan and Canada, announced their opposition to the current version of the plan following their meeting at the G20 summit of leading economies.
While acknowledging that the proposed deal contains "important elements that will be essential for a just and lasting peace," the leaders said it is only a "basis" for talks and "will require additional work".
They warned in a joint statement that borders "must not be changed by force" and raised concerns that the proposed limits on Ukraine's armed forces would leave the country vulnerable to future attack.
Among those signing the statement in Johannesburg were the leaders of Britain, Germany, France, Spain and Italy, plus top EU officials.
Ukrainian President Volodymyr Zelenskiy on Saturday appointed a delegation to negotiate with the US, Russia and other stakeholders.
Zelenskiy acknowledged the difficult position Ukraine faces.
He said Trump's plan presents Ukraine with "a very tough choice: either a loss of dignity or the risk of losing a key partner," referring to the US.
According to the Washington Post, the Trump administration is threatening to withdraw its support for Ukraine's defence against Russia's all-out invasion, if Kiev refuses to agree to the plan's terms.
Without US support - which provides essential weapons and intelligence - Ukraine's ability to continue fighting Russia would be severely constrained.
US Vice President JD Vance said that any plan to end Russia's war in Ukraine should preserve Ukrainian sovereignty and be acceptable to both countries but that it was a "fantasy" to think Ukraine could win if the US were to give it more money or weapons or impose more sanctions on Russia.
German Chancellor Merz said that the war cannot be ended on terms that Kiev has not agreed to.
"Wars cannot be ended by great powers over the heads of the countries involved," he said, adding that "an end to the war can, of course, only occur with Ukraine's unconditional consent."
He warned that a collapse of Ukraine would have consequences "for the entire European continent" and said that while there is currently an opportunity to end the war, Kiev and its allies remain "still quite a long way" from achieving a positive outcome.
Trump's plan would grant Russia broad territorial and military concessions, including requiring Ukraine to withdraw from areas of Donetsk and Luhansk - collectively known as the Donbass - it still controls.
It would also sharply reduce Ukraine's military to 600,000 personnel and require that a ban on NATO membership be enshrined in the constitution. NATO troops could also not be stationed in Ukraine.
The Donetsk and Luhansk regions, along with the Crimean Peninsula, which was illegally annexed by Moscow in 2014, would be recognised as de facto Russian territory.
In exchange for agreeing to these concessions, Ukraine was promised unspecified security guarantees from Washington. Ukraine would also be allowed to join the European Union.
Despite having set a deadline of November 27, Trump signalled on Saturday that his offer may not be final.
Asked whether the 28-point plan was his last word to Ukraine, he said: "No."
"We're trying to get it ended one way or the other," he added.
with Reuters
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