The US and five Arab allies have launched the first strikes against Islamic State (IS) militants in Syria.
The Pentagon said warplanes, drones and Tomahawk missiles were used in the attacks, which targeted several areas including IS stronghold Raqqa.
Syria's foreign ministry said its UN envoy was informed about the strikes against IS, which controls large swathes of Syria and Iraq. The US has already launched about 190 air strikes in Iraq since August.
However, Monday's action expands the campaign against the militant group across the border into Syria.
Pentagon spokesman Rear Adm John Kirby confirmed the operation, saying "US military and partner nation forces" had undertaken military action in Syria.
US Central Command (Centcom) said Sunni Arab countries Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates "participated in or supported" the strikes.