Syrian rebels say they have killed 20 army soldiers in a tunnel blast in the battle for the north-eastern suburbs of Damascus.
The operation took place before dawn on Sunday, said Wael Olwan, spokesman for the Failaq al-Rahman faction.
The Britain-based Syrian Observatory for Human Rights monitoring group says 16 soldiers are reported missing because of the blast.
A Russian-mediated ceasefire announced last month failed to end the fighting between the government and rebels in the area, with Failaq al-Rahman saying it is not party to the agreement, and the government saying it is fighting terrorists.
The government has relied on its air force and its ground-to-ground missile systems to push rebels out and away from the capital, and several neighbourhoods and towns have been destroyed in the process.
The opposition does not have an air force.
The Syrian government increased the size of the territory under its control by two and a half times in just two months, according to Russian defence minister Sergei Shoigu who predicted the end of the fight with Islamic State.
He said the fall of the militant stronghold Deir ez-Zor on the Euphrates will say a lot, if not everything, about the end of the battle with IS.
Russia is a close ally of the Syrian regime and has provided air support for Syrian forces since 2015.
IS has seen the territory it holds contract in recent months under pressure from Western-backed forces in Iraq and Syria.