Syrian President Bashar Assad, right, met with Russian Federation Council member Dmitry Sablin and the accompanying delegation from the Russian State Duma in Damascus earlier this month. (Sana Handout/EPA)

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.

Russia is a longtime ally of Assad’s government, and its September 2015 military intervention helped the Syrian army turn its war effort around. Moscow, which has been accused of indiscriminate attacks on civilians in opposition-held areas, is now a key player in diplomatic efforts to bring the war to an end.

Representatives from the opposition and Assad’s government head to Switzerland on Thursday for a fresh round of U.N.-backed peace talks, building on a nationwide truce announced by Russia and Turkey at the end of last year.

That cease-fire has looked increasingly shaky in recent days, however.

On Monday, the Syrian Observatory for Human Rights monitoring network said the Syrian government had escalated attacks on rebel-held suburbs of Damascus. Seven people were killed in bombing raids on the northern district of Barzeh, while rocket attacks on the opposition-held neighborhood of Qaboun continued throughout the day.

The White Helmets rescue brigade also published a video showing the apparent aftermath of a bombing raid on the rebel-held Damascus neighborhood of Tishreen. Rescue workers were seen digging a crying child from the bank of rubble that had been her home.

With all major urban centers now under government control, these final pockets of resistance are in the army’s crosshairs.

The Observatory reported Saturday that at least 16 people were killed in government rocket fire on a funeral in Qaboun. The neighborhood’s smuggling tunnels supply food and weapons to Eastern Ghouta, the rebel stronghold on the outskirts of Damascus that now poses the most direct threat to Assad’s hold on the capital.

On Sunday, Syria’s political opposition condemned the escalating violence as a “bloody message” aimed at sabotaging this week’s peace talks. A walkout by Eastern Ghouta’s largest rebel faction, Jaish al-Islam, would certainly cause significant problems.

“No opposition delegation would be of much value without the largest faction willing to participate,” Aron Lund, a fellow at the Washington-based Century Foundation, wrote in a research note last week.