Missiles struck Syrian military bases in Hama and Aleppo late Sunday in what state-run television called a "new aggression" by its enemies.
Syrian officials gave no details on who is responsible or if there were any damage or casualties.
But the Britain-based Syrian Observatory for Human Rights said the strikes, probably carried out by Israel, killed 26 pro-government fighters, mostly Iranians.
The Observatory said the rockets targeted a rocket warehouse at a base in Hama, as well as another base near Aleppo International Airport.
Israel is suspected of being behind an attack on a Syrian air base used by Iran earlier this month. Iran says seven Iranians were killed.
Israel has warned it would react forcefully anytime it believes an attack against its territory or aircraft is possible. It also blames Iran for arming Hezbollah militiamen.