BEIRUT, Lebanon — A suicide bomber detonated a truck filled with propane tanks at a crowded military checkpoint in central Syria on Sunday, killing more than 30 people, most of them civilians, in the second such attack by fighters linked to Al Qaeda in two days.

The attack, which was reported both by the state-run news media and by antigovernment activists, shook the city of Hama, ignited dozens of cars and sent up a column of smoke visible for miles around. One activist said the secondary explosions of bursting gas tanks had continued long after the initial blast.

Activists said the Nusra Front, one of the two Qaeda affiliates fighting alongside the rebels who seek to topple President Bashar al-Assad, was responsible for the attack.

The bombing followed a similar attack that killed 16 soldiers east of Damascus the day before, suggesting an increasing reliance on suicide attacks to try to break government strongholds that the rebels are unable to take by conventional means.