Putin 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," said Putin.

"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 has no plans to send troops to Ukraine but has 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 on Wednesday to a request for comment on 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 Putin's "nuclear sabre-rattling".

Mykhailo Podolyak, a senior Ukrainian presidential official, told Reuters in a statement he viewed Putin'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 Podolyak, who said he believed such talk showed Putin was afraid of losing the war.

However, speaking at a press conference in Tokyo on Thursday, the chief of the UN nuclear watchdog, Rafael Grossi, downplayed the possibility of a nuclear war.

"I don't think we see, at the moment, conditions for the use of nuclear weapons, if we are referring to the war in Ukraine," said Grossi, who met Putin last week.

The Ukraine war has triggered the deepest crisis in Moscow's relations with the West since the 1962 Cuban Missile Crisis. Putin has often warned of the risks of nuclear war but says he has never felt the need to use nuclear weapons in Ukraine.

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 is defending itself against an imperial-style war of conquest designed to erase its national identity. Putin says he sent tens of thousands of troops into Ukraine in February 2022 to bolster Russia's own security against a hostile West.

Putin 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," Putin said.

Putin'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 Putin is not ready for serious talks over Ukraine.

Reuters reported in February that 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.

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.

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," Putin said.

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