The Americans hit the region "without advance notice to Russia or Turkey", which both have troops on the ground in Idlib, the Russian military said. It described the attack as "indiscriminate".

The strike caused "great losses and destruction," the Russian defence ministry added in a statement, accusing Washington of having "compromised the ceasefire in the de-escalation zone of Idlib."

The US strike, which targeted leaders of Al-Qaeda in Syria, killed at least 40 jihadists, according to the Syrian Observatory for Human Rights.

It came as renewed Syrian regime bombardment of Idlib killed a civilian in the first violation of a Russian-backed truce for the region that came into effect just hours before.

Syrian government air strikes on the jihadist-run Idlib region had halted earlier Saturday, after the regime agreed to a Moscow-backed ceasefire following four months of deadly bombardment that killed more than 950 civilians, the monitor said.

Saturday's truce is the second such agreement between Syrian President Bashar al-Assad's regime and jihadists since August 1.

The Idlib region is home to some three million people, nearly half of whom have been displaced from other parts of Syria.

The United Nations says the violence there has displaced more than 400,000 people.