Putin launches nuclear drills as US says Russia poised to invade Ukraine

(Reuters) -Russia's President Vladimir Putin launched exercises by strategic nuclear missile forces on Saturday and Washington said Russian troops massed near Ukraine's border were "poised to strike".

As Western nations fear the start of one the worst conflicts since the Cold War, US Defence Secretary Lloyd Austin said Russian forces were beginning to "uncoil and move closer" to the border with its former Soviet neighbour.

"We hope he steps back from the brink of conflict," he told a news conference on a visit to Lithuania, saying an invasion of Ukraine was not inevitable.

Russia ordered the military build-up while demanding NATO stop Ukraine ever joining the alliance but says predictions it is planning to invade Ukraine are wrong and dangerous. It says it is now pulling back while Washington and allies insist the build-up is mounting.

Russian-backed separatist leaders in eastern Ukraine earlier declared a full military mobilisation, a day after ordering women and children to evacuate to Russia, citing the threat of an imminent attack by Ukrainian forces.

Kyiv flatly denied the accusation and Washington said it was part of Russia's plan to create a pretext for an invasion of Ukraine.

Multiple explosions could be heard on Saturday morning in the north of the separatist-controlled city of Donetsk in eastern Ukraine, a Reuters witness said. The origin was not immediately clear. Ukraine said earlier that one of its soldiers had been killed.

US President Joe Biden, who has given regular warnings of an impending invasion, said on Friday he now believes the capital Kyiv would be targeted by Russia but that he does not think Putin is even remotely contemplating using nuclear weapons.

"We have reason to believe the Russian forces are planning to and intend to attack Ukraine in the coming week, in the coming days," Biden told reporters at the White House.

"As of this moment, I am convinced that he has made the decision."

The exercises by Russia's nuclear forces on Saturday involve the launch of ballistic and cruise missiles, the defence ministry said. Kremlin spokesman Dmitry Peskov said the exercises were part of a regular training process and Putin was likely to take part from a "situation centre".

Russia's Ria news agency quoted the Kremlin on Saturday as saying the exercises had started.