US President Donald Trump told Fox News Radio he believed next Thursday was an appropriate deadline for Kyiv to accept the plan.
Trump later told reporters that time was short given the approaching winter and need to end the bloodshed and that Zelenskiy would have to approve the plan.
"He'll have to like it, and if he doesn't like it, then you know, they should just keep fighting, I guess," he said.
"At some point he's going to have to accept something he hasn't accepted."
Recalling his fractious February meeting with Zelenskiy, Trump added: "You remember right in the Oval Office, not so long ago, I said, 'You don't have the cards.'"
Washington's 28-point plan calls on Ukraine to cede territory, accept limits on its military and renounce ambitions to join NATO. It also contains some proposals Moscow may object to and requires its forces to pull back from some areas they have captured, according to a draft seen by Reuters.
Russia's President Vladimir Putin, who has refused to budge on Russia's key territorial and security demands, said on Friday the US plan could be the basis of a final resolution of the nearly four-year-old conflict.
In a sombre video statement, Zelenskiy pledged to work with the United States on the plan but said he expects more political pressure over the next week.
He urged Ukrainians to remain united in the face of what he said were additional attempts by Russia to derail the peace process.
"Now is one of the most difficult moments of our history. Now, the pressure on Ukraine is one of the heaviest," he said.
"Now, Ukraine can face a very difficult choice - either losing dignity or risk losing a major partner."
Three sources told Reuters that Ukraine was working on a counter-proposal to the 28-point plan with the United Kingdom, France and Germany.
"I will fight 24/7 to ensure that at least two points in the (US) plan are not overlooked - the dignity and freedom of Ukrainians," Zelenskiy said.
Russian forces are grinding forward on the battlefield and pounding Ukraine's energy system with missiles and drones as a domestic political crisis unfolds over a massive corruption probe involving senior Ukrainian officials and business elites.
The plan is expected to dominate discussions on the sidelines of the G20 summit in Johannesburg that European leaders are attending this weekend despite a boycott by Trump.
"We all want this war to end, but how it ends matters. Russia has no legal right whatsoever to any concessions from the country it invaded," said the EU foreign policy chief, Kaja Kallas. "This is a very dangerous moment for all."
US officials have said their plan was drafted after consultations with Rustem Umerov, secretary of Ukraine's National Security and Defence Council, a close Zelenskiy ally who served as defence minister until July.
Umerov "agreed to the majority of the plan, after making several modifications, and presented it to President Zelenskiy," a senior US official said. However, Umerov denied agreeing to any of the plan's terms and said he had played only a technical role organising talks.
The plan would require Ukraine to withdraw from territory it still controls in eastern provinces that Russia claims to have annexed, while Russia would give up smaller amounts of land it holds in other regions.
Ukraine would be permanently barred from joining the NATO military alliance, and its armed forces would be capped at 600,000 troops. NATO would agree never to station troops there.
Sanctions against Russia would be gradually lifted, Moscow would be invited back into the G8 group of industrialised countries, and frozen Russian assets would be pooled in an investment fund, with Washington given some of the profits.
One of Ukraine's main demands, for enforceable guarantees equivalent to NATO's mutual defence clause to deter Russia from attacking again, is dealt with in a single line with no details: "Ukraine will receive robust security guarantees".
23°C