Four Russian servicemen were killed by a roadside bomb in western Syria last week, according to the Russian military.

The defense ministry said in a statement that the Feb 16. attack had targeted a Syrian military convoy close to Tiyas, an air base from which Russian war planes have launched raids on Islamic State fighters in the ancient town of Palmyra.

It was not possible to immediately verify the Russian statement, and the attack has yet to be claimed by militants in Syria. But the announcement underscored the growing toll that Syria’s conflict has wrought on Russian military and mercenary forces fighting on behalf of Syrian President Bashar al-Assad.

Although Russian authorities acknowledge some combat deaths among military personnel, they often do so with a delay and without announcing an official tally.