The US and several Arab allies have launched the first strikes against Islamic State (IS) militants in Syria.
The Pentagon said fighter and bomber jets 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, who control 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 strikes, saying “US military and partner nation forces” were undertaking military action in Syria – but did not give details
Islamic State – also known as Isis, or Isil – has taken control of large areas of Syria and Iraq, imposed a harsh brand of Islam, and declared a caliphate.
The group, which the CIA says could have as many as 31,000 fighters in Iraq and Syria, has executed captive soldiers, aid workers and journalists, and threatened the mass killing of Iraqi religious minorities.
The IS advance in northern Syria has created a refugee crisis in neighbouring Turkey, with about 130,000 Kurdish refugees crossing the border at the weekend.
Most refugees are from Kobane, a Syrian town close to the Turkish border that is under siege by IS militants.
Before the latest influx, there were already more than one million Syrian refugees in Turkey.