(CNN) -- A bomb attack Sunday against military offices in a suburb of Damascus killed at least 31 Syrian troops, including four high-ranking officers, an opposition group said.
Rebel groups planted the explosives in Harasta, said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
A video posted on YouTube showed a massive explosion and the rapid collapse of a building. CNN could not independently verify the video or opposition account.
State-run media made no mention of the attack.
Syria's descent into civil war began in March 2011, when President Bashar al-Assad's regime forcefully cracked down on peaceful anti-government protesters.
That conflict spiraled into an armed uprising and a 2-year-old crisis that the United Nations says has claimed more than 100,000 lives.
CNN's Mohammed Tawfeeq and Samira Said contributed to this report.