Ukraine's pro-Moscow insurgents showed off their military prowess on Thursday by launching exercises involving heavy tanks that Kiev claims they had covertly received from Russia.

The self-proclaimed "tank biathlon" mirrored manoeuvres that Russia has been staging with its former Soviet allies since 2013.

"We want to show that we can do this as well as the Russians," tank commander Ruslan Zvenigorodsky told AFP.

"And we want to show the children watching that they should not be afraid of militants."

The 18-year old -- the word "Russia" emblazoned on his uniform -- said he had also joined battles across the eastern separatist provinces of Lugansk and Donetsk in the past 17 months.

But the peculiar military festivities were marred somewhat by an unexplained tank explosion about six kilometres (four miles) away in which one rebel fighter was killed.

"I have this feeling that this might be a terrorist act," separatist commander Eduard Basurin said.

The conflict -- Europe's bloodiest since the Balkans wars of the 1990s -- has killed nearly 8,000 people and effectively severed Moscow's relations with the West.

Russia hotly disputes fomenting the crisis in reprisal for last year's ouster in Kiev of a Moscow-backed leader and his eventual replacement by pro-EU President Petro Poroshenko.

But NATO head Jens Stoltenberg told AFP in Brussels there was "no doubt that there is a strong Russian presence in the eastern part of Ukraine".

Violence has subsided considerably since the warring sides signed off on a new truce deal on September 1. Kiev said one soldier was killed after setting off a tripwire but that otherwise the situation remained calm.

Yet tensions remain high and the sides have made little progress toward implementing a broader ceasefire and political reconciliation agreement that is due to go into force by the end of the year.

Both Kiev and rebel commanders have vowed to return to all out warfare should their political demands not be met at ongoing talks in the Belarussian capital of Minsk.

Poroshenko will also meet Russian President Vladimir Putin and the leaders of Germany and France in Paris on October 2 in a bid to get a broader consensus on clinching a lasting peace.

- 'Spectacular show' -

Organisers said at least 5,000 people had gathered for the first of three days of manoeuvres near a village some 50 kilometres (30 miles) east of the rebels' de facto capital Donetsk.

Thirty three T-72 tanks that insurgents say were captured from Ukrainian forces showed off their ability to plough through rough terrain and precisely fire shells.

"We were promised a spectacular show," said a local woman called Alyona after posing for a photograph with a machinegun in her hands.

"The fact that we have an event like this means that we are growing closer to the rest of the world," the 32-year-old said.

Militia leader Basurin said the entire event cost around two million rubles ($30,000/27,000 euros).

"Some of the money came from sponsors and partially from the republic's budget," he said without explaining who those financial backers might be.