Factbox: Russia's mounting deaths in Syria

(Reuters) - Reuters estimates that at least 40 Russian servicemen and private military contractors have been killed in Syria so far this year, four times the official death toll of 10.

Reuters reported an estimated total of 21 deaths in April.

Following are details of 19 more servicemen and contractors who have died, according to relatives, friends, former schoolmates, local officials and local and social media:

Families and friends of fallen soldiers and officials in their hometowns passed Reuters information about an additional five deaths since the April report: Gennady Perfilyev, a lieutenant colonel, special forces officer Bogdan Derevitsky, Aleksandr Skladan, Yevgeny Konstantinov and one more officer, whose name Reuters could not establish.

Private contractors to have died in Syria this year include Alexander Promogaibo, Andrei Volodin, Alexander Motynga, Yevgeni Chuprunov, Artyom Gorodilov, Yevgeni Tretyakov and a man called Dmitry from the Siberian city Novosibirsk, whose last name Reuters was unable to establish.

Two more individuals, Egor Anyokha and Prokopiy Solomonov from the republic of Yakutia in eastern Russia, were also killed in Syria, local officials and people who knew them said. Reuters could not determine if they fought with the army or with private contractors.

Names of servicemen whose deaths were confirmed by the Defense Ministry were marine commander Sergei Bordov, Igor Zavidny, Alexei Goncharenko, Alexei Buchelnikov and Nikolai Afanasov.