Ukraine has claimed that a Russian invasion is possible “at any minute”, amid reports that Moscow’s troops and armour were on the move in Crimea and a separatist leader in eastern Ukraine survived a bomb attack.

The warning from Kiev stoked fears of a return to all-out war in Ukraine, where the United Nations and monitors from the Organisation for Security and Co-operation in Europe (OSCE) have noted an ominous recent surge in violence.

The Russian forces that annexed Crimea in early 2014, following Ukraine’s pro-western revolution, stopped all traffic between the peninsula and the rest of Ukraine for several hours on Sunday. Local pro-Ukrainian activists said large numbers of Russian soldiers and armoured vehicles were in the area.

“The occupiers are conducting manoeuvres and we should understand that at any minute, at any hour, they could start a large-scale or small-scale attack,” said Andriy Lysenko, a military spokesman for Ukraine’s presidential administration.

On Saturday, a bomb exploded in the separatist-held city of Luhansk beside an SUV carrying the local militant leader, Igor Plotnitsky. The device appeared to be attached to a lamppost, which was split by the blast.

The extent of Mr Plotnitsky’s injuries was not clear, but separatist-run media released what they said was a recording made by him as he recovered in hospital, during which he accused Ukraine and its US backers of being behind the attack.

“This will be reported to the president of the Russian Federation and the FSB,” he said, referring to the security service formerly run by Russian president Vladimir Putin.

“Those who want remove the legal authorities in LNR [the so-called Luhansk People’s Republic] are provocateurs. It’s not worth listening to them, because they want to destabilise and reduce to nothing everything that we have achieved.”

Diplomatic support

Kiev denied involvement in the attack on Mr Plotnitsky, who has many local enemies following the assassination last year of several Cossack warlords in Luhansk region who had accused him of corruption and double-dealing.

Ukraine’s military intelligence agency said that following the assassination attempt, the separatists’ armed units in Luhansk were “conducting counter-sabotage measures and had gone to the highest level of combat readiness.”

A United Nations commission said last week that civilian casualties in eastern Ukraine were now at their highest level since last summer, due to a surge in artillery exchanges and other fighting over the last two months.

The OSCE monitors in the region have warned for several months that an escalation is taking place, as the sides take up positions inside a supposed “buffer zone” and engage at close quarters, while also using banned artillery from distance.

The rising violence has all but crushed lingering hopes for the so-called Minsk peace plan, most points of which have not been implemented by either side.