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, the Russian president, acknowledged battlefield setbacks for the first time 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 retaking Crimea by force impossible, because Russia was expected to fight tooth and claw to defend it.
But a senior US officer told The Telegraph that recent Russian military collapses mean “the recapture of Crimea by Ukraine is now a distinct possibility and can no longer be discounted”.
Pushing into Crimea would mean Ukraine going further than the front lines of Feb 23, when Putin ordered the invasion of Ukraine.
Russia annexed Crimea in 2014 and would likely view a ground assault on the critical strategic enclave 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 those from Laura Cooper, US deputy assistant secretary of defence, who said that 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 has described as "extremely dangerous" and "evidence of direct US involvement in the conflict".
On Wednesday, in a rare admission of difficulties at the front, Putin said: "We are working on the assumption that the situation in the new territories will stabilise.”
The Russian leader also signed a bill formalising Russia’s “annexation” of Luhansk and three other Ukrainian regions on Wednesday. He also ordered Russia’s state nuclear monopoly to take over the running of the Zaporizhzhia nuclear power plant.
On Wednesday Serhei Gaidai, the governor of Luhansk, announced that “the de-occupation of the Luhansk region has already officially started”.
Alexander Kots, a Russian journalist embedded with Russian forces there, revealed there were too few troops available to mount a credible defence against Ukraine's continuing advance.
The latest advance in the east came after the Russian front line in the south partially collapsed, leading to a retreat by 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”.
He said he had seen Russian soldiers shoot one of their own men in the back when he tried to surrender.
It came as Ukraine paraded the first newly mobilised soldier to have been captured fighting in Ukraine. The soldier said he had travelled to the front line with the intention of handing himself in, in a further sign of low morale.
Liz Truss, the Prime Minister, warned against compromising with Putin and said Britain would stand with Ukraine “however long it takes” for victory.
“We should not give in to those who want a deal which trades away Ukrainian land. They are proposing to pay in Ukrainian lives for the illusion of peace,” she told the Conservative Party conference.
“We will stand with our Ukrainian friends however long it takes. Ukraine can win, Ukraine must win, and Ukraine will win.
American government agencies said they would spend $290 million on anti-radiation drugs. Putin has previously hinted he would use nuclear weapons to avoid defeat in Ukraine.
Ukraine's lightning advance into the north-east on Wednesday revealed more alleged torture chambers used by Russian soldiers.