The blast came a day after Syrians marked the second anniversary of the start of their uprising against President Bashar Assad. The rebellion began with largely peaceful protests, but when the regime cracked down on demonstrators, the unrest evolved into an insurgency and then a civil war.
In recent months, the Assad regime has escalated airstrikes and artillery attacks on rebel-held areas in the north and east of the country, rights groups have said.
The Observatory also said at least 12 rebel fighters were killed in clashes near a cement factory in the northern city of Aleppo, and five people were killed when a shell exploded in the Damascus neighborhood of Qaboun.
Also Saturday, the head of Syria's leading opposition group issued an anniversary message to Syrians, saying that the uprising has "has taken a long time."
The opposition recognizes March 15, 2011 as the start of the uprising.
In a video posted on his Facebook page, Mouaz al-Khatib, head of the Syrian Opposition Coalition, congratulated the town of Yabrud, north of Damascus, for creating a civil council to run its affairs.
"Our people are great, our people are civilized and they don't need gangs to rule them," al-Khatib said, sitting in front of a Syrian flag and cracking a rare smile. "They just need to breathe a little bit of the air of freedom and they'll create as they have created in all places."
All videos appeared authentic and corresponded with other reporting by The Associated Press.
Associated Press writer Albert Aji in Damascus, Syria, contributed to this report.
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