The US has launched dozens of cruise missiles at an air base in Syria in response to a suspected chemical weapons attack on a rebel-held town.
More than 50 missiles were filed from US Navy destroyers in the eastern Mediterranean, striking several targets, a US official said, speaking on condition of anonymity.
Further details on the target and the results of the strikes were not immediately known.
US President Donald Trump is expected to address the nation shortly, CNN reported.
Earlier, US Secretary of State Rex Tillerson said Syrian leader Bashar al-Assad should have no role in a future Syria.
His comments signalled a sudden shift in policy by the new US administration.
The Syrian government has denied it was behind the deaths this week in the rebel-held town of Khan Sheikhoun.
At least 70 people, many of them children, died in the attack.