Unnamed military sources said army soldiers uncovered the 500-meter-long tunnel on Friday as they were evacuating militants and their families from the area under a deal reached between the Damascus government and foreign-backed Takfiri militant groups last month, Arabic-language and pro-government Jabal Amel news agency reported.
The sources added that terrorists used the tunnel to travel between the small mountainous town of Madaya, located about 40 kilometers (25 miles) northwest of Damascus, and Buqayn village in the Zabadani district of Rif Dimashq province.
The Syrian army's engineering units later placed explosives inside the tunnel and destroyed it.
Meanwhile, Lebanon-based Arabic-language al-Mayadeen news network reported on Friday that Syrian officials and militant groups have started to implement a new phase of the agreement for evacuations of four areas.
Ten buses carrying dozens of civilians left the militant-besieged Shia villages of al-Foua and Kefraya near the northwestern city of Idlib on Friday morning.
A convoy of four buses also moved armed men and their families from the town of Zabadani.
Under the deal reached between the Syrian government and militants in late March, the residents of Foua and Kefraya will be taken to the outskirts of Aleppo City, while the militants and their families will be transferred from Zabadani and Madaya near Damascus to militant-held territory in Idlib.
The evacuation process kicked off last Friday, but it came to a halt a day later when a bomber blew up an explosives-laden car ripping through several buses carrying evacuees from Kefraya and Foua.
The terrorist attack killed at least 126 people, including 68 children, and injured dozens of others.
The Syrian Arab Red Crescent (SARC) is supervising the evacuations, which is described as the biggest population swap of its kind.
The agreement also includes a prisoner swap, a ceasefire covering areas south of Damascus, and aid deliveries.