A 28-point plan that the United States has presented to Ukraine endorses some of Russia's demands and leaves security guarantees for Ukraine vague.
Two people familiar with the matter said the United States threatened to cut key support for Ukraine if it did not agree to the framework of the proposal.
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 United States has reportedly presented Ukraine with terms that include that it cede additional territory, curb the size of its military and be barred from joining the NATO military alliance.
Sanctions against Russia would be gradually lifted, it would be invited back into the G8 group of industrialised countries and frozen Russian assets would be pooled in an investment fund for Ukraine's reconstruction, with the US 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 the line "Ukraine will receive robust security guarantees".
A non-aggression pact between Russia and Ukraine and European countries is also reportedly part of the deal.
Zelenskiy said on Friday after a phone call with US Vice President JD Vance that Ukraine would work with the United States and European countries at an advisor level to work towards a peace plan.
"We agreed to work together with the US and Europe at the level of national security advisors to make the path to peace truly doable," he said on X after the nearly hour-long call.
"Ukraine has always respected and continues to respect US President Donald Trump's desire to put an end to the bloodshed, and we view every realistic proposal positively."
A US military delegation met Zelenskiy in Kyiv on Thursday.Â
The US ambassador and the army public affairs chief travelling with the delegation described the meeting as a success and said the United States sought an "aggressive timeline" for the US and Ukraine to sign a document.
Trump said in a radio interview that he thought Thursday was an appropriate deadline for Ukraine to accept the US-backed peace proposal.
"I've had a lot of deadlines but if things are working well, you tend to extend the deadlines. But Thursday is we think is an appropriate time," Trump told Fox News Radio on Friday.
President Vladimir Putin said on Friday Russia had received the US proposals and that the plan could be the basis of a peaceful resolution of the conflict.
"I believe that it can be used as the basis for a final peaceful settlement," Putin told senior officials in televised comments, adding that the plan had not been discussed in detail with Russia.
