Ukraine’s government has called on NATO for help after a battalion of Russian ground troops, including armoured vehicles, crossed the border into Ukraine.

Kiev claims that the Russians have set up a military headquarters near the village of Pobeda, around 50km southeast of the rebel held-city of Donetsk.

The incursion, which comes after several weeks of successful Ukrainian pushes deep into the last rebel strongholds, has prompted Ukraine to call on NATO for help.

Security spokesman Andriy Lysenko said the "tactical battalion" had dug in around the village, while five armoured personnel carriers and a truck had also been spotted entering the town of Amvrosiyivka, 25km northeast of Pobeda.

The incursion appears to have turned the tables back in the rebels’ favour, with insurgent fighters capturing large swathes of land from retreating government forces, according to reporters on the ground.

A Ukrainian volunteer commander posted on Facebook that government forces were surrounded in the key transport hub of Ilovaysk and reinforcements were desperately needed.

Ukraine's Prime Minister Arseniy Yatsenyuk has called on NATO to intervene and has said he will request assistance when the alliance holds a summit in Wales next week.

A member of Ukrainian volunteer battalion 'Dnipro' looks through binoculars near the small southern Ukrainian city of Novoazovsk. (AFP) (AFP)

Russia, which has always denied helping the insurgents, has not acknowledged the offensive, which has been condemned by world leaders.

“These incursions indicate a Russian-directed counteroffensive is likely underway,” US State Department spokeswoman Jen Psaki told reporters at the department’s daily briefing in Washington.

Ms Psaki has also criticized what she called the Russian government’s “unwillingness to tell the truth” that its military had sent soldiers inside Ukraine territory, the New York Times has reported.

Poland's Prime Minister Donald Tusk said NATO and Polish intelligence have evidence of regular Russian army units operating in Ukraine.

Between mid-July to mid-August, at least 1200 people have been killed in the region - more than double the total casualties in the conflict since it began in April, according to a report by UN rights monitors to be released last week.

The death toll does not count the 298 killed in the July 17 downing of Malaysia Airlines Flight MH17.

Smoke billows over fire after shelling on the outskirts of the small southern Ukrainian city of Novoazovsk. (AFP) (AFP)

NATO diplomats have also claimed a new Russian anti-aircraft missile system had been detected in a rebel-held area.

The SA-22 air defence system, which has a range of up to 20km, was "now in the zone", said an official, adding that Russian support of the rebels had become "more open" recently.

Russian Foreign Minister Sergei Lavrov said the Kremlin was "not interested in breaking up" Ukraine.

The latest incidents come after the Ukranian militatry captured several Russian soldiers who claimed to have accidently wandered over the border.

Moscow admitted some of its troops ended up on Ukrainian soil after Kiev revealled it had captured 10 Russian paratroopers 20km away from the border.

Russian President Vladimir Putin said the soldiers had been "patrolling the border”.