By Christian Lowe and Gabriela Baczynska

MOSCOW (Reuters) - Two Russian men captured by Ukrainian forces fighting pro-Russian rebels have told a newspaper they were in Ukraine on a mission for the Russian military, contradicting Moscow's official line.

Speaking from a hospital bed in Kiev, one of the men, Alexander Alexandrov, became tearful when his interviewer told him his relatives had told Russian state media that he had quit the Russian military before heading to Ukraine.

"Why are they turning their backs on me?" he was quoted as saying by the Russian weekly Novaya Gazeta. "There was an order. I gave my oath to the motherland ... There was an order and, as a military man, I carried it out."

Ukraine's military said the two men had been wounded in a firefight in eastern Ukraine, and were being treated for their injuries. Authorities in Kiev have said they will be charged with "terrorist acts".

In a video posted online by the Ukrainian Interior Ministry this week, Alexandrov said he had been on a spying mission in Ukraine as part of a 14-member special forces group from the Russian town of Togliatti.

Although Ukraine, NATO and Western leaders all say they have evidence that Russia is providing soldiers, training and weaponry to the pro-Russian rebels who have seized part of eastern Ukraine, Russia strenuously denies the charge.

But by insisting that any Russian fighters in the area are merely volunteers, Moscow finds itself unable to follow its own code of honor by standing up for servicemen who are captured.

Most Russians support the Kremlin's policy in Ukraine, but the fate of the two men has stirred debate about whether Russia is upholding its duty to protect those who serve it.

"This is awful. You cannot treat your own citizens this way," Ella Polyakova, from a Russian campaign group, the Soldiers' Mothers' Committee, told Reuters.

"It would be completely shameless to just abandon them, say they are 'volunteers' and let them take full responsibility," said Polyakova, also a member of a presidential advisory body on human rights.

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Russia's Defence Ministry has said Alexandrov and Yerofeyev served in the military in the past, but were no longer serving when they were captured.

Kremlin spokesman Dmitry Peskov has denied that there are any serving Russian soldiers in eastern Ukraine. He was quoted by the state-run RIA news agency as saying Moscow would take the necessary steps to free Alexandrov and Yerofeyev.

(Additional reporting by Michael Nienaber in Berlin; Editing by Kevin Liffey)