The US and Arab allies bombed Syria for the first time on Tuesday killing at least 70 of its fighters, a group tracking violence in the war has said.
Rami Addulrahman, who runs the Syrian Observeratory for Human Rights, told Reuters that the death toll was likely to be much higher.
US Central Command said Bahrain, Jordan, Qatar, Saudia Arabia and the UAE took part or supported the strikes against ISIL targets.
The targets of the air and missile strikes included the headquarters of ISIL in Raqqa city, northern Syria.
ISIL vowed revenge for the attacks.
Separately, the US acting alone launched strikes against the al-Qaeda-linked Nusra front.
The strikes hit targets in Aleppo and Idlib to “disrupt imminent attack” against US and Western interests by “seasoned al-Qaeda veterans.”
The Nusra Front is al-Qaeda’s official Syrian wing and ISIL’s rival.
The Syrian government had previously said that any military action taken without its consent would amount to an act of aggression.