Russia's military on Tuesday said that a soldier had killed himself while on duty in Syria, the first confirmed death of a serviceman in the war-torn country since Moscow launched its bombing campaign last month.

The defence ministry in Moscow did not identify the soldier who it said committed suicide but said the serviceman appeared to have taken his life due to problems with his girlfriend.

The official announcement came after two people close to the family of a young soldier serving in Syria told AFP that he had died from unknown causes while on duty in the country, with one of the sources casting doubt on the idea that he might have killed himself.

AFP could not immediately verify if that serviceman, identified as 19-year-old Vadim Kostenko, a career soldier from the southern Krasnodar region, was the same as the one whose death was confirmed by the defence ministry.

The government's quick confirmation of a death of a serviceman is hugely unusual and comes as world attention is focused on Russia's bombing campaign in its Soviet-era ally Syria.

"A contract soldier who served as a technical specialist at the Hmeimim airbase committed suicide while resting after his shift," a defence ministry spokesperson said in a statement carried by Russian news agencies.

Citing preliminary reports, the spokesperson said that text messages found on his phone suggested he took his own life because of relationship problems.

A person close to Kostenko's family, who spoke to AFP by phone from Krasnodar, said that the cause of his death was unclear and that he would be buried this week.

Grigory Donskikh, who served with Kostenko in Russia and knew him well, said the young soldier had gone to Syria of his own free will and he did not believe he had committed suicide.

"He is not someone who would take his own life," he told AFP, adding his body had not arrived home yet.

Deaths from mishandling of weapons, hazing and suicides are not uncommon in militaries worldwide, and have been a persistent issue in the Russian army.

But experts say that the Russian defence ministry may also cite a suicide to conceal the true reason of a soldier's death.

In an interview with Reuters at their home in southern Russia, Alexander and Svetlana Kostenko said their son, Vadim, had sounded cheerful over the phone as recently as Saturday, the day he died while working at an air base on the Syrian coast.

"I will never believe this version (suicide)," said Svetlana, who was wearing a black head scarf. "We spoke every day by phone for half an hour. (On Saturday) he was cheerful, happy, and he laughed," she said.

Alexander, Vadim's father, speaking in a low voice, agreed: "We were told he had hanged himself because of a girl. He would never have done it. I know my son really well."

Kostenko was one of the Russian air force's support staff. He signed a contract on June 20 and was dispatched to Syria by plane on Sept. 14, two weeks before the Kremlin's air campaign began, his father said. He said they had only discovered Vadim was in Syria when he was already there.