Government troops "managed to open a road" on Sunday to the Military Vehicles administration base in the city of Harasta, in the Eastern Ghouta district, freeing some 200 troops trapped inside the compound, the British-based monitor said.
The government forces had been besieged by rebel fighters from the Ahrar al-Sham group and al-Rahman Corps since an offensive on December 31, during which anti-government forces expanded their control over the site.
Some 160 pro-government and rebel fighters have been killed in fighting over the base since December 31, according to SOHR.
Rebel forces stormed the site in November 2017 in an attempt to prevent government strikes on rebel-held enclaves in Eastern Ghouta.
Government assaults on the district have been frequent in recent weeks and are believed to be part of the Syrian government's strategy to retake rebel-held positions.
At least 17 civilians were killed in aerial bombardments in the area on January 6, taking the total number of civilian casualties in Eastern Ghouta to 103 - including 47 women and children - since December 29, according to the SOHR.
Al Jazeera has not been able to independently verify the figures.
Hamish de Bretton-Gordon, an adviser to a coalition of medical charities operating in Syria, told Al Jazeera from the UK city of Salisbury that more than 120 children were in need of urgent medical care in the Eastern Ghouta district, noting that the last week and a half has been "horrific in terms of the amount of attacks".
Eastern Ghouta is one of a handful of so-called de-escalation zones in Syria, in which military activity is prohibited under a ceasefire agreement reached last year by Turkey, Russia and Iran.
The area is one of the last rebel strongholds in the country and is home to some 400,000 people. A four-year government siege has led to a humanitarian crisis, with severe shortages of food and medicine.
Car bomb attack
Elsewhere in Syria, at least 23 people, including seven civilians, were killed in a car bomb attack on Sunday in the country's northwestern city of Idlib, according to the SOHR.
The blast, for which there has been no claim of responsibility so far, struck the military headquarters of the Ajnad al-Kavkaz armed group, the monitor reported.
The monitor, which gathers its information from a network of sources inside Syria, added that it was not clear whether the attack was specifically targeting the group's base.
The rebel-held province of Idlib has seen increased violence in recent weeks as Syrian President Bashar al-Assad intensifies his efforts to gain control over the region.
Government forces have captured more than 60 villages in Idlib province in the past two weeks, according to the SOHR.