Ramzan Kadyrov, the Chechen leader and key Kremlin ally, has called on Vladimir Putin to use tactical nuclear weapons against Ukraine in the wake of another major and embarrassing military defeat.
His comments marked the first time a Russian official has openly and explicitly called for the use of atomic bombs in Ukraine.
It came after Moscow admitted it had withdrawn from the key eastern city of Lyman in the face of "significant superiority in forces and means", handing Kyiv a major battlefield victory.
Video posted online shows Ukrainian troops entering the city in the north of Donetsk, a region President Vladimir Putin illegally annexed just the day before.
In the footage, two grinning Ukrainian soldiers are seen raising the country's blue and yellow flag by the welcome sign at the entrance to the city.
"October 1. We're unfurling our state flag and establishing it on our land. Lyman will be Ukraine," one of the soldiers said, standing on the bonnet of a military vehicle.
Russia's defence ministry announced that its troops had withdrawn "to more advantageous lines" to avoid encirclement in Lyman, which had been a logistics and transport hub for Russian forces in the occupied Donbas region.
It claimed without evidence that it had inflicted heavy casualties on advancing Ukrainian forces by launching "massive fire strikes" against them.
But it also admitted that Ukraine had gained the upper hand. "Despite the losses suffered, the enemy, having a significant superiority in forces and means, introduced reserves and continued the offensive in this direction," it said.
The announcement came a day after Putin proclaimed that Donetsk and three other Ukrainian regions were now part of Russia "forever".
"I want to say this to the Kyiv regime and its masters in the West: People living in Lugansk, Donetsk, Kherson and Zaporizhzhia are becoming our citizens forever," President Putin said during a ceremony at the Kremlin to announce the annexation of the Ukrainian regions.
The humiliating defeat prompted his loyal footsoldier, Kadyrov, to urge an intensification of the war.
"In my personal opinion, more drastic measures should be taken, right up to the declaration of martial law in [Russia's] border areas and the use of low-yield nuclear weapons," Kadyrov wrote in a post on the Telegram messaging app.
Until now, Russian leaders have only obliquely threatened the use of nuclear weapons.
In an address last week, Putin said he was not "bluffing" when he said Russia had "various weapons of destruction" at its disposal and would use "all the means available".
Russia has the world's largest atomic arsenal, including low-yield tactical nuclear weapons that are designed to be deployed against opposing armies.
The Kremlin announced that any attack on annexed Ukrainian territory would be treated as an attack on Russia.
Senior British Conservative MPs said the threat should be taken seriously.
Duncan Smith, the former Tory leader who is a candidate to be the next chair of the foreign affairs select committee, told The Telegraph: "He wouldn't be saying this if he hadn't been authorised by Putin."
He said Russian military tactics had always considered nuclear weapons to be "war-fighting weapons, whereas in the West we see them solely as weapons of escalation and therefore we wouldn't use them".
"I wouldn't underestimate the possibility that he could use these weapons, that's not just an empty threat," he said, adding that Putin was "trying to up the stakes" to "get the West to back down".
He noted that in response, the West must "double down" on its military and economic support to Ukraine.
"The only way to deal with this is to stand up to him and his surrogate and say, 'if you use these weapons, we will not back down," he said.
Duncan Smith added that the West should also make clear that anyone involved in deploying nuclear weapons would be pursued by "the swords of justice".
Tobias Ellwood, the chair of the defence select committee, said that allies including the UK had to "agree on our kinetic conventional response to Putin detonating a single tactical nuclear weapon in Ukraine".
Ellwood said such a "grave threshold cannot be crossed unpunished", but "neither should we be spooked by the increased rhetoric".
The United States has directly warned the Kremlin that Russia will face "catastrophic consequences" if it deploys nuclear weapons in Ukraine.
Jake Sullivan, the US national security adviser, said last week the US had "communicated directly, privately to the Russians at very high levels" how it would respond in the event of Moscow using atomic bombs in Ukraine.
"If Russia crosses this line, there will be catastrophic consequences for Russia. The United States will respond decisively," Sullivan said.
Kadyrov, who has been critical of Russia's progress in its invasion, blamed the defeat in Lyman on the local commander, Colonel General Sergei Lapin.
"The Colonel General deployed mobilised fighters of the LPR [Luhansk People's Republic] and other units on all fronts near Lyman, but did not provide them with the necessary communications, coordination and ammunition supplies," Kadyrov said, accusing the general of being "nowhere near" his troops.
Serhiy Cherevatyi, a spokesman for Ukraine's eastern forces, said the victory would undermine President Putin's goal of capturing the industrial Donbas region.
"Lyman is important because it is the next step towards the liberation of the Ukrainian Donbas. It is an opportunity to go further to Kreminna and Severodonetsk, and it is psychologically very important," he said.
Lyman's liberation represents Kyiv's biggest victory since a major counter-offensive in the northeastern Kharkiv region last month. From Lyman Ukrainian forces will look to make further advances into the adjacent Luhansk province.
The Ukrainian military claimed to have encircled thousands of the Kremlin's troops in Lyman – using loudspeakers to broadcast messages calling on surrounded soldiers to surrender.
Cherevatyi said that previously there were "around 5000 to 5500" Russian troops in the area, but military action could have "reduced" their numbers. It is not known what happened to the soldiers.
At least one Russian convoy withdrawing from the city was destroyed by Ukrainian forces, according to a video shared online.
"Russian convoy was trying to get out of encirclement unsuccessfully, taking their looted things with them," said Anton Gerashchenko, a Ukrainian government adviser.
Pro-Moscow military bloggers claimed Russian forces had withdrawn in a planned retreat.
"By the middle of the day it became obvious that a decision had been made to withdraw troops and withdraw from Lyman," Rybar, a prominent Russian military blogger, wrote on Telegram.
A series of recent battlefield humiliations for Russia has raised concerns that a cornered Putin could resort to desperate measures.
The 69-year-old strongman, who over two decades in power has stifled all opposition, is facing mounting criticism of the invasion at home, with hundreds of thousands of Russians fleeing the country in recent weeks after Putin announced a partial mobilisation in response to recent losses.
The site was used by Hezbollah to plan attacks on Israeli civilians.