MOSCOW, Sept 03, 2014 (AFP) - Russian photographer Andrei Stenin, missing since early August, has been confirmed dead in Ukraine, the Russia Today media group he worked for announced on Wednesday.

Moscow immediately demanded a complete and impartial investigation into the "barbaric killing" of the 33-year-old journalist with those responsible brought to justice. "Our colleague, the photographer Andrei Stenin, is dead. It turns out he was not a prisoner, he has been dead for a month," said the staunchly pro-Kremlin media group's head Dmitry Kiselev. "His car was hit by shots and it had been burnt on the road close to Donetsk," Mr Kiselev said, referring to a rebel stronghold in eastern Ukraine.

Russia had opened a probe in mid-August into Mr Stenin's disappearance and it had been believed that he was taken captive by the Ukrainian authorities. "Unfortunately, when we were all trying to do our best, and even the impossible, to save him, Andrei was already no longer alive," said Mr Kiselev.

Mr Stenin had been on assignment in Ukraine since May 14 for Rossiya Segodnya (Russia Today), a state-held holding company which includes RIA-Novosti news agency and TV news channels including the English language station RT.

With three journalists already killed in eastern Ukraine, Russian media had rallied behind efforts to find and free Stenin. News broadcasts on all national channels and many websites prominently displayed "Free Andrei" banners.

But the Investigative Committee, Russia's version of the FBI, said Wednesday that it believes that Mr Stenin was killed on the evening of Aug 6.

It said that Mr Stenin had been travelling along with a convoy of refugees that came under fire from Ukrainian tanks and armoured personnel carriers.

The Investigative Committee said genetic analysis of partial remains recovered from the site determined that they belonged to Mr Stenin.

The Russian Foreign Ministry deplored the "shocking circumstances" of Mr Stenin's "barbaric killing" and called for a full and impartial investigation with those responsible handed severe penalties.

President Vladimir Putin offered his condolences to Stenin's family and friends and praised the journalist for the sacrifice he made to report on the conflict.

"He did everything so people, the whole world, knew the truth about the tragic events taking place in Donetsk," Mr Putin said in a letter to Mr Stenin's mother released by the Kremlin.

"He carried out his journalistic and moral duties to the end," added Mr Putin.

Mr Kiselev said Mr Stenin's remains would be transported to Moscow on Wednesday from the southern Russian city of Rostov where the identification work had taken place.

The funeral has been scheduled for Friday.

Three other Russian journalists were killed in Ukraine in June.

An Italian photographer and his Russian assistant were killed in late May.