On Sunday, the militants fired rockets and artillery shells in the towns of Kafr Nabl and Zahra, killing 20 Syrians and injuring many others.
The dead included many women and children.
The militants fighting government forces have carried out similar attacks in recent months.
Meanwhile, there are reports of successful advances by the Syrian army.
The military has managed to regain control of several strategic towns and locations over the past few days.
A large number of insurgents have reportedly been killed in the clashes with the army.
The Syria crisis began in March 2011, and many people, including large numbers of soldiers and security personnel, have been killed in the violence.
The Syrian government says that the chaos is being orchestrated from outside the country, and there are reports that a very large number of the militants are foreign nationals.
In an interview recently broadcast on Turkish television, Syrian President Bashar al-Assad said that if the militants take power in Syria, they could destabilize the entire Middle East region for decades.
“If the unrest in Syria leads to the partitioning of the country, or if the terrorist forces take control… the situation will inevitably spill over into neighboring countries and create a domino effect throughout the Middle East and beyond,” he added.