A former US national security adviser has warned Vladimir Putin could "lash out" with nuclear weapons following a series of embarrassing withdrawals for the Russian army.
Ukraine claims to have regained more than 6,000 square kilometres of land from Russian control in recent days, one of its biggest victories since the war began.
Territorial advances have been so significant Kyiv has even claimed that, in north-east Ukraine, Russian troops have been pushed back as far as the border.
Moscow has claimed its retreat has been a tactical withdrawal but it represents a significant setback for Russia.
The British ministry of defence said on Monday that rapid Ukrainian successes have had "significant implications for Russia's overall operational design".
"In the face of Ukrainian advances, Russia has likely ordered the withdrawal of its troops from the entirety of occupied Kharkiv Oblast west of the Oskil River," an intelligence update said.
However, a note of caution has been sounded by former US national security adviser John Bolton, who has said that while Putin using nuclear weapons had previously been "extremely unlikely", Ukrainian advances could change that.
"My view has been that it was extremely unlikely - and the only circumstances I could foresee where he would use battlefield or tactical nuclear weapons, [was] if Russian troops are in wholesale retreat heading back to the borders of Russia," Bolton told LBC.
"And we're not at that point after this most recent Ukraine offensive - but, it's been a military catastrophe for the Russian army and I do think, to try and reestablish some aspect of a position of strength to the Russians, it does become somewhat more likely now of a possible nuclear use."
Similar warnings have also come from a former Nato chief.
Rose Gottemoeller, an American diplomat who served as Deputy Secretary General of Nato from October 2016 to October 2019, told the BBC: “I fear that they will strike back now in really unpredictable ways and ways that may even involve weapons of mass destruction."
Moscow has raised the prospect of utilising its nuclear capability in recent weeks.
In August, Putin acknowledged that "no one" could win a nuclear war, stating: "We proceed from the fact that there can be no winners in a nuclear war and it should never be unleashed, and we stand for equal and indivisible security for all members of the world community," he said.
However, later that month, Kremlin spokesman Dmitry Peskov outlined that Moscow would be willing to launch a potentially devastating strike if it posed an "existential threat" to the country.
"We have a concept of domestic security, and it's public. You can read all the reasons for nuclear arms to be used," he said. "So if it is an existential threat for our country, then it can be used in accordance with our concept."
There have been some signs that Russia's previously tight grip on the internal narrative is also beginning to fray.
On Sunday, a former state Duma deputy Boris Nadezhdin claimed it is "absolutely impossible" for Russia to defeat Ukraine.
Appearing on the state-owned NTV channel, he said Russia's invasion of Ukraine was failing and suggested there should be "peace talks".
On Monday, dozens of municipal deputies from Moscow and St. Petersburg called on Putin to resign in an open letter published Monday in a further signal of dissent.
On the ground, Russia has already launched retaliatory strikes in response to Ukraine's military progress by targeting critical infrastructure like energy grids.
Ukrainian president Volodymyr Zelenskyy said Russian Federation "terrorists" were trying to deprive Ukrainians of light and heat.
"A total blackout in the Kharkiv and Donetsk regions, a partial one in the Zaporizhzhia, Dnipropetrovsk and Sumy regions," Zelenskyy tweeted on Sunday night. "Russian Federation terrorists remain terrorists and attack critical infrastructure. No military facilities, the goal is to deprive people of light and heat."
Watch: Russian strikes cause 'total' blackout in Kharkiv after Ukraine's shock gains in counteroffensive