The Ukrainian military said on Wednesday that more Russian soldiers had crossed the border into eastern Ukraine, entering the small town of Amvrosiyivka in five armored infantry carriers and a truck.
The report of a further cross-border incursion by Russian troops followed the capture by Ukrainian government forces of a group of Russian paratroopers in roughly the same area.
Moscow said the captured troops had strayed into Ukraine by mistake but Kiev said they had been on a "special mission" linked to the pro-Russian separatist rebellions in the east.
"Five armored infantry carriers and one Kamaz truck entered Amvrosiyivka with men in them," military spokesman Andriy Lysenko told journalists in Kiev.
"If this tactical group got lost and accidentally came into Ukraine like the paratroops of the 98th paratroop division then it remains for us to remind them that they can return to Russia, taking an easterly direction," Lysenko remarked.
Fighting was meanwhile continuing in key settlements near the rebel-held city of Donetsk despite peace talks on Tuesday in Minsk between Ukrainian President Petro Poroshenko and Russia's Vladimir Putin at which the Ukrainian leader pledged to work on a road map that would lead to a ceasefire.
Over the past 24 hours, Lysenko said, there had been intense fighting further north near the town of Horlivka and Ilovaysk, about 50 km (30 miles) away in which 200 separatists had been killed and their tanks and missile systems destroyed.
Thirteen Ukrainian service personnel had been killed in the past 24 hours in 34 clashes at points across eastern and south-eastern Ukraine.
Ukrainian military officials say the separatists, backed by Russian soldiers and armed with armor and military equipment coming from across the border, have switched the focus in the war towards the south-east of Ukraine down to the Azov Sea.
Military successes there for the separatists could put in danger the key port city of Mariupol, held at the moment by government forces. If separatists got a grip on the south-east they could reinforce their positions in Donetsk from the south.
Lysenko said that rebels "together with the Russian occupiers" had taken the settlement of Starobesheve, south-east from Donetsk, destroying a hospital in the process. Wounded, he said, were being evacuated south.
The video footage of the captured Russian paratroopers is the strongest evidence yet to support Kiev's claims of Russian involvement in the five-month conflict in which the United Nations says more than 2,000 people - troops, civilians and rebels - have been killed.
The crisis, which followed the ousting of a Moscow-backed president in Ukraine by street protests and a take-over by a pro-Western leadership has sparked the worst confrontation between Russia and the West since the end of the Cold War.
Separately, Prime Minister Arseny Yatseniuk on Tuesday said Ukraine needed "practical help" from the U.S.-led NATO alliance, with which it has the status of a special partner, and expected it to take "momentous" decisions in this regard at its summit in Wales in early September.
"We need help," Yatseniuk told a government meeting.
Though Ukraine is not a NATO member and does not qualify for direct military support from the alliance, NATO has taken Kiev's side in the conflict, blaming Russia for creating the crisis by annexing Crimea in March and fuelling the rebellions in the east and arming the separatists.