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 announcement came after two people, including a person close to the family of a young soldier serving in Syria, told AFP 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 focussed 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.

The spokesperson said that text messages found on his phone suggested he took his own life because of relationship problems.

-'Not someone to take his life' -

A person close to Kostenko's family, who spoke to AFP by phone from Krasnodar, said that the cause of his death was unclear, adding that he had learned of Kostenko's demise on Saturday.

Grigory Donskikh, who served with Kostenko in Russia, went further, saying he did not believe he had committed suicide.

"He is not someone who would take his own life," he told AFP.

Kostenko is expected to be buried in the Krasnodar region on Wednesday.

Relatives and friends mourned his death in posts on VKontakte, Russia's most popular social network.

"We remember, we love, we grieve," a young woman identified as Katya Kostenko, Vadim's sister, wrote on her page, alongside a photo of a uniformed soldier clutching a Kalashnikov.

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.

"We have repeatedly seen a situation where a family of a dead soldier is told that their son committed suicide ostensibly because of relationship problems," Anna Kashirtseva, spokeswoman for the Mother's Right group, told AFP.

Genuine suicides are rare, with servicemen instead coming under "physical and psychological pressure" in most of the cases, she said.

The defence ministry did not respond to a request for comment from AFP.

On September 30, Russia launched a bombing campaign in Syria, saying it needed to target Islamic State militants.

Russian warplanes have carried out sorties from the Hmeimim airbase and Russian servicemen are based there to help service the aircraft, among other tasks.

Russia insists that its ground troops are not taking part in combat operations in Syria and there have been no official reports of any servicemen dying in combat in the war-torn country.

-'Either illness or trauma'-

A member of the Kremlin's rights council, who advocates for the rights of soldiers and works with the defence ministry, said he was not aware of Kostenko's death.

But the activist, Sergei Krivenko, said that his defence ministry sources told him "about 10 days ago" that a Russian soldier had died "at the base" in Syria.

"It was either an illness or trauma," Krivenko, the head of the Citizen and the Army group, told AFP.

On Friday, The Wall Street Journal reported that one Russian soldier had died in Syria "due to careless handling of weaponry."

Officials have previously denied accusations from Kiev and the West that it sent regular troops to eastern Ukraine to prop up pro-Kremlin insurgents.

The state enforced a virtual blackout on reports of Russian soldier deaths in Ukraine despite reports to the contrary including funerals of soldiers.

Earlier this year, the Kremlin signed off on a law making all military deaths a state secret.

Russia's entry into the multi-front conflict in Syria is Moscow's first major military involvement outside the former Soviet Union since the occupation of Afghanistan in 1979.