BEIRUT — An explosion tore through a secret meeting of one of Syria’s strongest and most enduring rebel groups on Tuesday, killing a dozen of its top leaders, including its head, and striking another blow against the forces seeking to topple President Bashar al-Assad.

The blast hit a basement where the leaders of the group, Ahrar al-Sham, had collected to plot strategy, according to antigovernment activists. It remained unclear who had carried out the attack, which reportedly killed dozens of people and occurred in Idlib Province in Syria’s north.

The explosion added to the troubles facing Syria’s rebels, who have lost ground in the country’s civil war in recent months to Mr. Assad’s military while also being overshadowed by the Islamic State in Iraq and Syria, the jihadist group that has seized territory on both sides of the Iraq-Syria border.

After three years of declining to intervene in Syria’s civil war, President Obama began airstrikes against ISIS positions in Iraq last month and is working to form an international coalition to battle the group, which calls itself the Islamic State.