Syrian rebels say they have captured a government helicopter base near Damascus as they try to close in on President Bashar al-Assad's seat of power in the capital.

A video released by Syrian activists on Sunday appears to show anti-Assad rebels roaming around the Marj al-Sultan air base, about 15 kilometers east of Damascus, after they launched an assault the previous day. The Britain-based Syrian Observatory for Human Rights said the rebels killed at least eight soldiers and destroyed two helicopters at the base, while at least 15 rebel fighters also were killed.

There was no government confirmation of the report.

Activists say Syrian opposition forces trying to end President Assad's autocratic rule have been gradually expanding their territory around Damascus, despite being heavily outgunned by government forces who remain entrenched in the capital.

In other developments Sunday, the Observatory said rebels seized a training base of a pro-Assad Palestinian militant faction near Damascus. The Popular Front for the Liberation of Palestine condemned the attack on its base, which it said has been used to train thousands of Palestinians to fight Israel in the past 30 years.

Elsewhere, the Observatory said a bomb attack on a bus killed at least five people in southern Syria.

Syria's government describes the rebels as foreign-backed terrorists. The rebellion began in March 2011 with pro-democracy protests against Mr. Assad before evolving into a civil war. It is difficult to verify details of the fighting independently because Syria tightly restricts reporting by foreign journalists.