Russia's force in Syria has suffered losses since late January more than three times higher than the official toll, according to evidence gathered by Reuters, a tally that shows the fight in Syria is tougher and more costly than the Kremlin has disclosed.

Eighteen Russian citizens fighting alongside Moscow's allies, the Syrian government forces, have been killed since January 29—a period that coincided with intense fighting to recapture the city of Palmyra from the Islamic State militant group (ISIS).

The Russian defense ministry has publicly reported only five servicemen's deaths in Syria over the same period, and its officials' statements have not mentioned any large-scale Russian ground operations in the fight for Palmyra.

Trending: Michael Dorf: What We Are Learning About Judge Gorsuch

Military casualties abroad are not as politically sensitive in Russia as in some other countries but send a negative message ahead of a presidential election next year that is expected to give President Vladimir Putin a fourth term.

The toll was revealed in interviews with relatives and friends of the dead men, cemetery workers, local media reports of funerals and evidence collected by a group of investigative bloggers, Conflict Intelligence Team (CIT).

In each case, Reuters has independently verified information about the death by speaking to someone who knows the dead man.

The casualties since the end of January represent one of the highest tolls for the Russian contingent in Syria since the start of Moscow's military intervention 18 months ago.

An official with the Russian foreign ministry refered questions about them to the defense ministry. The Russian defense ministry did not respond to Reuters questions about the casualties and about military operations in Syria. The Kremlin did not immediately respond to a request for comment.

Most of the dead were not regular Russian soldiers but Russian civilians working as private military contractors under the orders of Russian commanders. Moscow has not officially acknowledged the presence of the contractors in Syria.

Story continues

Don't miss: Election Investigation Finds Republican Worried About Illegal Voting Committed Voter Fraud

One of the 18 men killed was Yuri Sokalsky, a 52-year-old from the Russian Black Sea resort of Gelendzhik who, according to a person close to him, signed up to go to Syria in January with a group of private contractors.

In one of his last phone calls home, the person close to him said, he expressed surprise at the large numbers of Russian contractors being despatched to Syria, and relayed what he had been told about the intensity of the combat.

"Out of every 100 people, 50 are coming back in caskets," the person recalled Sokalsky as saying. The person asked not to be identified, fearing repercussions for revealing information that is sensitive for the Russian authorities.

On March 14 last year, Russian President Vladimir Putin announced a partial drawdown of Russian forces in Syria, saying their mission had "as a whole, been fulfilled." The fight for Palmyra this year tells a different story.

The 18 fatalities documented by Reuters include the five regular soldiers whose deaths were announced by the defense ministry, four private military contractors in one unit killed on the same day, seven other such contractors, and two regular soldiers whose deaths the defense ministry has not announced.

The period examined by Reuters coincided with the start of a major Russian deployment to the area around Palmyra, according to several people close to the dead fighters.

Most popular: Guterres and Haley Can’t Do It Alone, Democracies Must Stand Up To End U.N. Farce

Several relatives of people killed in Syria said they had received phone calls from people involved in recruiting private military contractors warning them not to speak to media.

Out of the 18 dead, at least 10 were killed in the region of Palmyra, which ISIS fighters seized in December for a second time in a year—a major reversal for Syrian government forces and their Russian backers.

On January 10, Sokalsky, a land mine specialist, left his home in Gelendzhik and set off for Rostov, in southern Russia, to join a group of private contractors being despatched to Syria.

On his one previous tour to Syria, only fighters over 35 were being hired, selected to carry out specialist technical roles or train Syrian units rather than for out-and-out combat.

"This time they were taking everyone," said the person close to Sokalsky.

Two official documents seen by Reuters show that on January 31, Sokalsky died from shrapnel injuries in Tiyas, in Syria's Homs province about 37 miles west of Palmyra. Three other members of his unit, all private military contractors, were killed the same day, according to relatives, friends and cemetery officials. They were Alexei Nainodin, Roman Rudenko, and a third man whose name Reuters was not able to establish.

Another private military contractor, Dmitry Markelov, was also killed at Tiyas, site of the Syrian military's T4 air base, on January 29, according to people close to him.

Four regular Russian servicemen were killed in the same area on February 16, Russian state media cited a defense ministry statement as saying. The soldiers, described by state media as "advisors" to the Syrian military, were not named. A fifth regular serviceman, Artyom Gorbunov, was killed near Palmyra on March 2, state media quoted the ministry as saying.

A further eight members of the Russian contingent were killed since the end of January at unknown locations in Syria, the evidence gathered by Reuters showed.

They were contractors Konstantin Zadorozhny, Ivan Slyshkin, Vasily Yurlin, Alexander Sagaydak, Alexander Zangiyev and Alexander Tychinin, and regular Russian soldiers Igor Vorona, and Sergei Travin.

Local media reports and social media posts point to more Russian deaths in Syria since the end of January than the 18 casualties, but Reuters has not been able to verify that information independently.

More from Newsweek