Ukraine's security service on Wednesday announced the capture of a Russian army major who was allegedly delivering explosives to pro-Kremlin rebels in the former Soviet state's separatist east.

Moscow denies any involvement in Ukraine's 15-month conflict and describes Russians caught in the war zone as "volunteers" who joined the fighting without any direct orders from the army.

But both Kiev and its Western allies accuse Russia of sending in troops and heavy weapons to support the insurgency.

The National Security Service of Ukraine (SBU) identified the Russian allegedly captured in the separatist Donetsk region at the weekend as Vladimir Starkov -- an army major who got lost while driving a truck filled with explosives to a guerrilla outpost.

"Starkov immediately confessed to being a Russian citizen and to being an enlisted soldier of the Russian armed forces," SBU chief Vasyl Grytsak told reporters.

Ukraine in May showed off two captured Russians who identified themselves as acting servicemen sent on a reconnaissance mission by their military intelligence command.

Russia claims the two had quit the force by the time they joined the fighting in the former Soviet state.

The website of Ukraine's Novoye Vremya weekly on Wednesday published the transcript of what it said was a conversation between two pro-Russian insurgents who had just learned of Starkov's detention.

"How did he get lost," asked a fighter whom the purported radio intercept identified as Malysh (Baby).

"He had taken that route a hundred times already!"

Russian President Vladimir Putin's spokesman on Wednesday reaffirmed Moscow's denial of involvement in the conflict and refused to be drawn on Ukraine's latest claim.

"We have repeatedly issued comments about such things," Interfax quoted the Kremlin spokesman as telling reporters in Moscow.

"I have nothing to add to what we have repeatedly said."