President Vladimir Putin launched exercises by Russia’s strategic nuclear missile forces on Saturday and Washington said Russian troops massed near Ukraine’s border were moving forward and “poised to strike”.

With western nations fearing what could be one of 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 (Putin) steps back from the brink of conflict,” Austin told a news conference in Lithuania, saying an invasion of Ukraine was not inevitable.

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

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

Kiev denied the accusation. It and western leaders say the mobilisation, evacuation and increased shelling across the ceasefire line this week are part of a Russian plan to create a pretext for an invasion of Ukraine.

Russia’s FSB security service said two shells landed on Russian territory near the border, Russia’s Tass news agency reported. One hit a building in Rostov region but no one was hurt, it said.

Ukraine’s foreign minister denied any Ukrainian shells had hit Russian territory and demanded an independent international investigation of the alleged incidents.

Ukraine’s military accused Russia of faking pictures of shells to make out they were Ukrainian, and said mercenaries had arrived in separatist-held eastern Ukraine to stage provocations in collaboration with Russia’s special services.

“The purpose of these provocations will, of course, be to accuse Ukraine of further escalation,” it said.

Multiple explosions were heard on Saturday in the north of the separatist-controlled city of Donetsk in eastern Ukraine, as more people got on buses to leave, a Reuters witness said. The origin of the explosions was not immediately clear. Ukraine said one of its soldiers had been killed.

“It’s really scary. I’ve taken everything I could carry,” said Tatyana, 30, who was boarding a bus with her four-year-old daughter.

US President Joe Biden said on Friday he believed the capital Kiev would be targeted by Russia but that he did not think Putin was contemplating using nuclear weapons.

Biden said Putin would invade in the coming days: “As of this moment, I am convinced that he has made the decision.”

Russian-backed rebels seized a swathe of eastern Ukraine and Russia annexed Crimea from Ukraine in 2014 after protests toppled Ukraine’s pro-Russian leader. Kiev says more than 14,000 people have been killed in the conflict in the east.

In one breakaway region, Denis Pushilin, head of the self-proclaimed Donetsk People’s Republic, said he had signed a decree on mobilisation and called on men “able to hold a weapon in their hands” to come to military commissariats.