Russian President Vladimir Putin has told the West that Russia was technically ready for nuclear war and that if the US sent troops to Ukraine, it would be considered a significant escalation of the conflict.
Mr Putin, speaking ahead of a 15-17 March election which is certain to give him another six years in power, added that the nuclear war scenario was not "rushing" up and he saw no need for the use of nuclear weapons in Ukraine.
"From a military-technical point of view, we are, of course, ready," Mr Putin, 71, told Rossiya-1 television and news agency RIA when asked whether Russia was really ready for a nuclear war.
He said the US understood that if it deployed American troops on Russian territory - or to Ukraine - Russia would treat the move as an intervention.
Moscow claims to have annexed four regions of Ukraine and says they are now fully part of Russia.
"(In the US) there are enough specialists in the field of Russian-American relations and in the field of strategic restraint," Mr Putin said.
"Therefore, I don't think that here everything is rushing to it (nuclear confrontation), but we are ready for this."
The Biden administration has said it had no plans to send troops to Ukraine but stressed the need to approve a stalled security aid bill that would ensure Ukrainian troops got the weapons they need to continue the war, now in its third year.
It did not immediately respond to a request for comment on Mr Putin's remarks, but the White House has said in the past it has seen no sign that Russia is preparing to use nuclear weapons despite what it calls Mr Putin's "nuclear saber-rattling".
Mykhailo Podolyak, a senior Ukrainian presidential official, told Reuters in a statement he viewed the Russian president's nuclear warning as propaganda designed to intimidate the West.
"Realising that things are going the wrong way, Putin continues to use classic nuclear rhetoric. With the old Soviet hope - 'be scared and retreat!'," said Mr Podolyak, who said he believed such talk showed Mr Putin was afraid of losing the war.
The war in Ukraine has triggered the deepest crisis in Moscow's relations with the West since the 1962 Cuban Missile Crisis.
Mr Putin has often warned of the risks of nuclear war but says he has never felt the need to use nuclear weapons in Ukraine.
Nuclear doctrine
In a US election year, the West is grappling with how to support Kyiv against Russia, which now controls almost one-fifth of Ukrainian territory and is rearming much faster than the West and Ukraine.
Kyiv says it was defending itself against an imperial-style war of conquest designed to erase its national identity.
Mr Putin said he sent tens of thousands of troops into Ukraine in February 2022 to bolster Russia's own security against a hostile West.
He reiterated the use of nuclear weapons was spelled out in the Kremlin's nuclear doctrine, which sets out the conditions under which it would use such a weapon: broadly a response to an attack using nuclear or other weapons of mass destruction, or the use of conventional weapons against Russia "when the very existence of the state is put under threat".
"Weapons exist in order to use them," Mr Putin said.
The Russian president's nuclear warning came alongside another offer for talks on Ukraine as part of a new post-Cold War demarcation of European security.
The US says Mr Putin is not ready for serious talks over Ukraine.
Reuters reported last month that Mr Putin's suggestion of a ceasefire in Ukraine to freeze the war was rejected by the US after contacts between intermediaries.
US Central Intelligence Agency Director William Burns said this week that, without more Western support, Ukraine would lose more territory to Russia which would embolden Chinese President Xi Jinping.
Mr Burns, a former US ambassador to Russia, told the Senate Intelligence Committee it was in US interests to help Kyiv get into a stronger position before talks.
Mr Putin said Russia would need written security guarantees in the event of any settlement.
"I don't trust anyone, but we need guarantees, and guarantees must be spelled out, they must be such that we would be satisfied," he said.
Battlefield situation 'difficult' - Ukraine
Ukraine's army chief said the situation on the battlefield was "difficult" and that Russian forces could be poised to strike deep into Ukrainian lines in the eastern Donetsk region.
Kyiv's forces are on the defensive across the 1,000-kilometre front lines in the east and south after Moscow made its first territorial gains in almost a year.
Commander-in-Chief Oleksandr Syrsky said he had visited two brigades "where the situation is gradually becoming more complicated and there is a threat of enemy units advancing deep into our battle formations".
"In general, the operational situation on the eastern front remains difficult. The enemy continues to conduct offensive actions," in a number of areas of the Donetsk region, Mr Syrsky said in a post on Telegram.
"At the same time, probably due to the high level of losses, the activity of the enemy in other areas of the front decreased significantly," he added, without elaborating.
In an interview with French media on Monday, Ukrainian President Volodymyr Zelensky said Russia's advance had been "halted" in the east and that the situation was "much better" than it had been three months ago.
Russian forces captured the symbolic frontline town of Avdiivka, just outside the Russian-held city of Donetsk, in February after one of the bloodiest battles of the two-year war.
They have since claimed to have captured a number of small settlements further to the west as their troops seek to press their advantage.
A Ukrainian army spokesman rejected a claim issued by Moscow yesterday that it had captured the village of Nevelske in the eastern Donetsk region.
Russia's claims to have seized the village "don't correspond to reality... there are no grounds to say that (Ukraine's) control over Nevelske has been lost", Dmytro Lykhoviy, a spokesman for Ukrainian forces in the area, told AFP.
At least two killed in Russian attacks on Ukraine's eastern regions - Kyiv
At least two people were killed in overnight Russian drone and bomb attacks in Ukraine's eastern Sumy and Donetsk regions, local officials have said.
Russians dropped a bomb on Myrnohrad town in Donetsk region, killing two and injuring five people, local governor Vadym Filashkin said on Telegram messaging app.
The Sumy regional military administration said there were casualties as a result of a Russian drone hitting an apartment block overnight.
The administration said 30 apartments of a five-storey residential building were damaged, 15 of them largely destroyed.
Ten people were rescued from the rubble, eight of whom sustained injuries, the officials said.
Late yesterday, Russian missile slammed into two apartment buildings in the central Ukrainian city of Kryvyi Rih, killing four people and injuring at least 50.
President Zelensky, who was born and raised in the city, praised rescue teams on Telegram and vowed Russia would be brought to account.
