A pro-Russian rebel leader has claimed that between 3,000 and 4,000 Russian citizens are fighting with separatists in eastern Ukraine as a Kiev official accused Moscow of a "full-scale invasion".

Alexander Zakharchenko revealed that the Russian citizens were former servicemen for the Russian army or military personnel on leave.

Zakharchenko made the declaration as rebels opened up a new front on Ukraine's southern coast in the town of Novoazovsk, approximately 12 miles from the Russian border. Fighting is also continuing around the separatist-held city of Donetsk in eastern Ukraine.

Mykhailo Lysenko, deputy commander of the Ukrainian Donbas Battlion, said that the rebels were being supported by Russian tanks and armoured personnel carriers.

On the new fighting in the south, Lysenko said: "This is a full-scale invasion".

The revelation that thousands of Russians are fighting alongside pro-Russian separatists comes after Nato's secretary general announced that the military alliance is set to dispatch forces to new eastern European bases, in response to the ongoing Ukraine crisis and to counter the threat posed by Russia to post-Soviet states.

"We will adopt what we call a readiness action plan with the aim to be able to act swiftly in this completely new security environment in Europe," he said.

"We have something already called the Nato Response Force, whose purpose is to be able to be deployed rapidly if needed. Now it's our intention to develop what I would call a 'spearhead' within that Response Force at very, very high readiness.

No comment was available from the Russian defence ministry on the accusations of an incursion into Ukraine, and of thousands of Russian citizens fighting with rebel forces.

The massing of thousands of Russian troops on Ukraine's border and reports of Russian incursions into Ukraine have put a spotlight on Moscow's intentions towards the post-Soviet states bordering Nato member nations.

UN agencies estimate that more than 2,000 people have been killed in the fighting between pro-Russian separatists and the Ukrainian military since April.