The death toll among Russian forces in Syria during a period of intense fighting to retake the city of Palmyra now stands at 21, according to evidence gathered by Reuters, after information emerged about the deaths of three military contractors.

The Reuters tally over the period from Jan. 29 until the end of March is more than four times higher than the official toll given by the Russian defence ministry of five servicemen killed. Russian forces are backing Syrian President Bashar al-Assad in his war with rebels and militants seeking to oust him.

Reuters reported last month, based on conversations with friends and relatives of the killed men, social media posts, and cemetery officials, that 18 Russian citizens had been killed from Jan. 29 to March 31.

Since then, Reuters has established the deaths in combat of a further three - Alexei Safonov, Vladimir Plutinsky and Mikhail Nefedov - that interviews with people close to them indicated they were military contractors rather than regular servicemen.

Apart from regular soldiers, Russia deploys private contractors to Syria. They have civilian status, officially, but are often retired veterans with battlefield experience. They act as an attack force in ground operations under military command, sources familiar with the operation in Syria have previously told Reuters.

   Moscow does not publicly admit the presence of military contractors in Syria and does not reveal casualties among them. The official total Russian military death toll since Moscow's 2015 intervention in Syria is 30, but may be considerably higher as under Russian law military casualties are a state secret.

The Russian defence ministry, the foreign ministry, and the Russian consulate in Damascus did not respond to requests for comment on the three contractors submitted by Reuters.

The family of 41-year-old Safonov, who died in mid-March, was told little about how it happened, one of his relatives said on condition of anonymity.

"I can only say one thing: it was in Syria," said the relative. Safonov had fought in eastern Ukraine, where pro-Moscow rebels are fighting a separatist war, before moving on to Syria, according to his relative.

Safonov was buried in his home village of Khutor Pochtovy, southern Russia, two weeks after his death. 

Several of Safonov's neighbours, and a local official, confirmed that he had died in Syria.

Pyotr Ivanov, head of the local draft board, did not confirm the circumstances of his death but said he went to the funeral because Safonov was a veteran of the military's past campaigns against a rebellion in the Russian region of Chechnya. He was not a serviceman when he died, Ivanov told Reuters.

Plutinsky, a 46-year-old resident of a settlement called Stanitsa Kazanskaya in southern Russia, was killed in Tiyas near Palmyra on Feb. 12, according to an official document that had data from his death certificate and was seen by Reuters.

A relative of Plutinsky confirmed that he had died in the Syrian province of Homs, which includes Tiyas, and said he had not been serving in the army at the time.

Nefedov, 27, a private military contractor, was killed in Syria in February, an acquaintance, Alexander Pashkov, told Reuters, confirming earlier reports in Russian media about Nefedov's death.