Ukraine could retake Crimea if it continues to recapture territory from fleeing Russian forces at its current rate, senior US military officials believe.
The assessment came as Vladimir Putin for the first time acknowledged battlefield setbacks and Ukrainian troops pursued the Russian army into the Luhansk region, reversing one of the Kremlin's key gains of the war.
Western officials have previously considered the retaking of Crimea by force as impossible because Russia was expected to fight tooth-and-claw to defend it. But a senior US officer told The Daily Telegraph that recent Russian military collapses mean "the recapture of Crimea by Ukraine is a distinct possibility and can no longer be discounted".
Pushing into Crimea would mean Ukraine going further than the front lines of February 23, when Putin ordered the invasion of Ukraine.
Russia annexed Crimea in 2014, and would probably view a ground assault on it as a major escalation.
The US official said: "It is clear that Russia no longer has the ability or willpower to defend key positions, and if the Ukrainians succeed in their goal of recapturing Kherson, then there is a very real possibility that it will ultimately be able to recapture Crimea."
The comments follow Laura K Cooper, the US deputy assistant secretary of defence, saying Crimea was within Ukraine's grasp. "And just to be clear, Crimea is Ukraine," she added.
She also said US weapons could be used to strike Crimea - a move the Kremlin yesterday described as "extremely dangerous" and "evidence of direct US involvement".
In a rare admission of difficulties at the front, Putin said yesterday: "We are working on the assumption that the situation in the new territories will stabilise."
He also signed a bill formalising Russia's "annexation" of Luhansk and three other Ukrainian regions yesterday and ordered Russia's state nuclear monopoly to take over the running of the Zaporizhzhia nuclear power plant.
Alexander Kots, a Russian journalist embedded with forces there, revealed that there were too few troops available to mount a credible defence against Ukraine's continuing advance.
The latest advance came after the Russian frontline in the south partially collapsed, leading to a retreat for at least 15 miles in the southern Kherson region on Tuesday.
A Ukrainian soldier fighting there told The Telegraph that Russian forces were "running away" and abandoning "rusty weapons".
Meanwhile, British Prime Minister Liz Truss warned yesterday against compromising with Putin and said Britain "will stand with our Ukrainian friends however long it takes".
"Ukraine can win, Ukraine must win, and Ukraine will win," she said.