The US is pressuring Ukraine to be open to peace talks with Russia, with a top Pentagon official saying that Kiev's forces will find it hard to recover all the territory Russia has controlled during the war.

US Joint Chiefs Chairman General Mark Milley said on Wednesday that the US and allied support has not diminished but considered that Kiev's defense puts it in a better position to begin talks.

"The probability of a Ukrainian military victory, defined as kicking the Russians out of all of Ukraine, to include ... Crimea, the probability of that happening anytime soon is not high, militarily," he indicated.

"There may be a political solution where, politically, the Russians withdraw, that's possible," he added, pointing out that "you want to negotiate from a position of strength. Russia right now is on its back."

Intelligence chiefs meet

White House National Security Spokesperson, John Kirby, claimed on Friday that the US is not trying to force Kiev to hold talks or cede territory.

Kirby told reporters that only Ukrainian President Volodymyr Zelensky "gets to determine if and when he's ready for negotiations and what those negotiations look like."

"Nobody from the United States is pushing, prodding or nudging him to the table," he said.

But earlier this month, Zelensky dropped his precondition that Russian President Vladimir Putin had to be out of power before he agreed to talks -- a shift, the Kyiv Post reported, that came after White House pressure.

In the same context, CIA Director William Burns held on Monday talks with Russian SVR intelligence Chief Sergei Naryshkin in Ankara, the highest-level in-person meeting of US and Russian officials since the war began in February. 

The details of their meeting remain secret, but Burns flew to Kiev to meet with Zelensky immediately afterward. Later, the White House claimed that Burns "is not conducting negotiations of any kind . . . We firmly stick to our fundamental principle: nothing about Ukraine without Ukraine."

When there's an opportunity to negotiate, seize it: Milley

Last week, Milley said in New York that Ukraine has lost nearly 100,000 in dead and wounded on the battlefield and some 40,000 civilian casualties, suggesting that the number could multiply if Kiev insisted on trying to fight all the way to its pre-2014 borders.

The US Joint Chiefs Chairman said one million people were killed over August-December 1914, the first months of World War I, after which the front lines were cemented into place, yet neither side would negotiate, and that "turned into 20 million killed by 1918."

"So when there's an opportunity to negotiate, when peace can be achieved, seize it," he indicated.

In response, Zelensky told the Halifax International Security Forum on Friday that "any voiced ideas of our land's concessions or of our sovereignty cannot be called peace. Immoral compromises will lead to new blood."

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