The first Russian serviceman confirmed dead in Syria was laid to rest in his native village on Wednesday as his family and friends disputed the official explanation that he committed suicide.

Authorities said 19-year-old soldier Vadim Kostenko hanged himself at Russia's airbase in the Syrian province of Latakia due to relationship trouble, but his loved ones said they would never believe he had taken his own life.

Several hundred people attended the funeral at the small village of Grechanaya Balka in southern Russia, including soldiers, defence officials and several journalists as international attention focuses on Moscow's bombing campaign in Syria.

During a funeral procession, four servicemen carried a wooden coffin from Kostenko's home to a cemetery down a muddy road, a soldier at the front holding a Russian flag with a black ribbon.

Friends and family wept as the casket was lowered into the ground with a three-shot gun salute.

Afterwards mourners laid wreaths and flowers at the grave as Kostenko's distraught mother clutched a framed picture of her son, who had recently ended his mandatory military service and rejoined the army on a contract.

- 'Not a suicide'-

The main military prosecutor's office said Wednesday that the autopsy results showed the cause of death as "suicide by hanging while off duty due to problematic personal circumstances outside the base."

But family and friends said they did not believe this version of events.

The soldier's mother was too distraught to comment so it fell upon his 14-year-old sister to brief the media.

"His body arrived last night. We saw his nose and jaw were broken, he had marks on his neck," Yekaterina Kostenko told AFP inside the family home, a brick house at 102 Soviet Street, before the funeral.

"This was not a suicide," said Kostenko, a blue-eyed girl with freckles and a long brown braid.

Svetlana Shpag, 42, a former colleague of Kostenko's mother, added: "No one believes the suicide story. He couldn't have killed himself for a girl. He wasn't one of those suicidal kids."

"The only reason he could have killed himself is if someone was about to take his life."

A defence ministry source said a priest had come earlier in the day but could not lead a service as per Orthodox tradition because the death was being treated as a suicide.

The defence ministry made the announcement after two people, including a person close to Kostenko's family, told AFP the soldier had died from unknown causes.

Experts say Moscow often cites a suicide to conceal the true reason of a soldier's death.

Genuine suicides are rare, with servicemen instead coming under "physical and psychological pressure" in most of the cases, Anna Kashirtseva, spokeswoman for the Mother's Right group, told AFP.

A defence ministry spokesman said that any suggestions pointing to possible foul play were "insinuations."

- 'They should apologise' -

People close to Kostenko's family said the young man had been encouraged by some not to go Syria and demanded an apology from the government.

"The people responsible for this should come here and apologise," the soldier's former gym teacher Stepan Bagryuk told AFP.

Kostenko was based at the Primorsko-Akhtarsk military air base not far from the village, but visited home whenever he could and was well-liked, Bagryuk said in the schoolyard where Kostenko played as a boy.

"I last spoke to him in August and he was fine."

On September 30, Russia launched its bombing campaign in Syria, with President Vladimir Putin saying Moscow needed to target Islamic State militants.

After the intervention -- Moscow's first major military foray outside the Soviet Union since the ill-fated Afghanistan occupation in 1979 -- Putin saw his approval rating climb to 89.9 percent, an all-time high, said a state pollster.

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

Moscow says 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.