KIEV: Ukraine on Tuesday began hearing a case against two Russian soldiers it claims will prove Moscow's direct involvement the conflict in the war-torn former Soviet state.

Troops captured the two men, both of whom were wounded, in May and interrogated them, broadcasting their alleged confessions in which they said they were active members of a Russian military intelligence unit operating in Ukraine.

But Moscow claims the two -- Captain Yevgeny Yerofeyev and Sergeant Aleksander Aleksandrov -- are "volunteers": retired servicemen who joined pro-Russian militias fighting in Ukraine's separatist east since early last year.

After being treated for their injuries, the two soldiers were then transferred to pre-trial detention on charges of "terrorism" and waging an "aggressive war" against Ukraine.

Prosecutors are seeking to have them jailed for life, their lawyer said.

"We will do everything to make sure that our citizens return home as soon as possible and get a chance to see their relatives and friends," Russian consul Alexei Gruby told reporters.

Sitting in the defendants' cage at Kiev district court as the charges were read out, both looked healthy although Yerofeyev's arm was still bandaged following an injury sustained during the gun battle which led to their capture in the industrial province of Lugansk.

"No one left us abandoned. We remain in contact with our family," Yerofeyev told reporters during a brief break in the preliminary hearings.

He said prosecutors were "pressuring" them into giving testimony favourable to their case, and said his parents were not present because "it was not safe".

Kiev has long accused Russia of both orchestrating and backing the eastern insurrection in the weeks that followed its March 2014 annexation of Ukraine's strategic Crimea peninsula.

Ukrainian President Petro Poroshenko paints the conflict -- a brutal battle that has killed nearly 8,000 and devastated the strategic rust belt -- as Russia's retribution for the February 2014 ouster of Kiev's Moscow-backed leadership.

That development shattered Russian President Vladimir Putin's ambition of reuniting the former Soviet republics into a single political and economic union with the clout to challenge Washington and Brussels.

Analysts believe Putin has recently changed his mind about seizing Ukraine's coal mine and steel mills because of the immense cost involved in rebuilding the ruined and destitute region.