A number of rockets hit a building in the Damascus district of Jaramana causing several casualties, Syria's state media said early on Sunday. It blamed "terrorist groups."
However, government loyalists on social media said the explosions were an Israeli strike believed to have killed Lebanese militant Samir Kuntar, who is reviled in Israel for a 1979 attack that killed four people.
Israel released Kuntar in 2008 as part of a prisoner swap with the Lebanese Shi'ite Hezbollah group.
Some Syrian government loyalists said he was in the building at the time of the attack but his fate was unclear.
It was not immediately possible to confirm the reports.