A reporter with a top Moscow opposition paper, that has won numerous awards in the West, was detained Tuesday in separatist eastern Ukraine and is likely to be expelled to Russia, his colleagues said.

Novaya Gazeta has been one of the only Russian media sources to investigate Western and Kiev accusations that the Kremlin has been both orchestrating and supporting the 14-month uprising in the industrial heartland of its ex-Soviet neighbour Ukraine.

The paper's editor and several reporters said journalist Pavel Kanygin was detained in a rebel-run part of the Ukrainian war zone and taken to a separatist-controlled village on the border with Russia.

"It seems like he was detained by the (rebels') state security ministry," separatist military commander Eduard Basurin told AFP by telephone.

A Russian foreign ministry spokeswoman said Moscow was aware of the reporter's detention and was seeking his immediate release.

"All the necessary measures will be taken to defend Russian journalists," ministry spokeswoman Maria Zakharova told Moscow radio.

A reporter with Moscow's Kommersant daily tweeted that Kanygin had told him by telephone "that it seems like he is going to be handed over to agents of (Russia's) Federal Security Service (FSB)."

The service is the domestic arm of what was once the Soviet-era KGB. Vladimir Putin -- a former spy in east Germany who headed the FSB before becoming president -- has denied any Russian involvement in Ukraine.

Novaya Gazeta editor Dmitry Muratov said his reporter was shortly due to be handed over to Russian agents stationed on their side of a Ukrainian border crossing.

"We are very happy about that," Muratov told Russia's RIA Novosti state news agency.

Muratov had earlier told Moscow Echo radio that the reporter was detained by rebel security agents who accused him of "working with certain Ukrainian media".

Both Moscow and Kiev -- along with the rebels -- have been waging a furious public relations campaign designed to win over both global and domestic opinion to their cause.

Novaya Gazeta has developed a reputation for fearless reporting about human rights allegedly committed by the Kremlin and its Russian allies.

Its reporter Anna Politkovskaya was shot dead near the entrance of her Moscow apartment building after conducting a series of investigations into alleged torture practised by the Moscow-appointed chief of the volatile southern region of Chechnya,

Both Politkovskaya and the paper itself have won awards for bravery in Europe and the United States.