State security chief Vasyl Hrytsak told reporters that Vladimir Starkov, 37, from Russia's Kirov region, had admitted immediately he was a serving soldier in the Russian armed forces after he was stopped in a truck at a checkpoint 22 km (14 miles) outside the separatist-held city of Donetsk.
Ukraine is likely to use the case to bolster its charges that Russia is continuing its involvement in the 15-month-long conflict and undermining a peace agreement worked out in Minsk, Belarus, in February.
While supporting the separatists' cause, the Kremlin denies it is supplying them with arms and equipment and that its forces are engaged in the conflict in Ukraine's east.
When Ukraine captured two Russian soldiers in May, Russia said the two men had quit their special forces unit to go to Ukraine of their own volition.
In a video released by the SBU state security agency, Starkov said that after arriving for service in Russia's Rostov region he was ordered to go to Ukraine as a military adviser to the rebels.
"They (the commanders) place you before an accomplished fact that you will serve in the DNR or the LNR (rebels' Donetsk or Luhansk people republics)," Starkov said.
SBU officials say Starkov and another man who said he was a separatist fighter lost their way and driven towards Ukrainian forces manning the checkpoint.
An SBU official told Reuters that Starkov had been accused of terrorism.
A fragile ceasefire, though punctuated by occasional clashes, largely seems to be holding while the sides withdraw heavy weapons from a buffer zone.
But each side accuses the other of failing to honor the Minsk agreements.
More than 6,600 people have been killed in the conflict.
Speaking in the western city of Lviv on Wednesday, President Petro Poroshenko repeated that all Russian forces had to be withdrawn from Ukraine.
"Russian forces must get out of Ukraine's territory.
State sovereignty must be renewed in the uncontrolled part of the Ukrainian-Russian border," he said.
Source: Google News