A peace agreement on Ukraine might be reached in a few months, and not in 24 hours, as U.S. President-elect Donald Trump had promised, Reuters reports citing informed sources.

"Advisers to President-elect Donald Trump now concede that the Ukraine war will take months or even longer to resolve, a sharp reality check on his biggest foreign policy promise - to strike a peace deal on his first day in the White House," it said on Wednesday.

According to the publication, two Trump associates, who have discussed the war in Ukraine with the president-elect, told Reuters they "were looking at a timeline of months to resolve the conflict, describing the Day One promises as a combination of campaign bluster and a lack of appreciation of the intractability of the conflict and the time it takes to staff up a new administration."

Those assessments dovetail with remarks by Trump's incoming Russia-Ukraine envoy, retired Lieutenant-General Keith Kellogg, who said in an interview with Fox News last week that he would like to have a "solution" to the war within 100 days, far beyond the president-elect's original timeline.

Yet even Kellogg's extended deadline was "way, way too optimistic," said John Herbst, a former U.S. ambassador to Ukraine who is now at the Atlantic Council think tank in Washington.