"Yesterday (Monday), the Americans informed the Syrian representative at the United Nations that strikes would be carried out against the terrorist IS organisation in Raqa," the group's Syrian stronghold, a foreign ministry statement quoted by state television said.
The television reported multiple strikes on Raqqa province, in the Euphrates Valley northeast of the capital.
US media reported that five Arab allies had joined in the strikes -- Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates.
The Pentagon did not immediately confirm where the strikes took place, but activists reported some 20 strikes against IS targets in Raqa province.
Raqa is the militant main bastion in Syria but they also control most of Deir Ezzor province further east, as well as sections of Aleppo in the north and Hasakeh in the northeast.