US forces attacked militant leaders in north-western Syria on Sunday, said the Pentagon, in what a battlefield monitor called a missile strike that left at least 40 dead.
The US Defence Department said the attack targeted leaders of Al Qaeda in Syria north of Idlib. Details of the attack were not disclosed, according to an AFP report.
The missiles targeted leaders of militant groups and allied factions near Idlib, the Syrian Observatory for Human Rights said.
Syrian government airstrikes on the militant-run Idlib region had stopped on Saturday after the regime agreed to a Russia-backed ceasefire following four months of deadly bombardment.
But "a missile attack targeted a meeting held by the leaders of Hurras Al Deen, Ansar Al Tawhid and other allied groups inside a training camp" near Idlib city, said Rami Abdul Rahman, head of the Observatory.
Kills Militant Leaders
The attack killed at least 40 militant leaders, the Britain-based monitor said.
The US Central Command said in a statement that the attack targeted leaders of Al Qaeda in Syria "responsible for attacks threatening US citizens, our partners and innocent civilians. Additionally, the removal of this facility will further degrade their ability to conduct future attacks and destabilise the region."
An AFP correspondent saw clouds of black smoke rising over the area after blasts rocked the militant stronghold. Ambulances rushed to the site of the attack, which was closed off to journalists, he said.
It was not immediately clear if the missiles were launched from warplanes or positions on the ground.
The truce that came into effect on Saturday is the second such agreement between the Syrian regime and militants since an August 1 ceasefire deal covering the Idlib region broke down only days after going into effect.
Russia-backed regime forces have been pressing an offensive against the major opposition stronghold in Idlib since April. But Russia and Damascus are not the only players with a history of strike activity in the area.
On July 1, the US said it had carried out a strike on Hurras Al-Deen in northwestern Syria, in its first such operation there in two years.
Hurras Al-Deen was established in February 2018 and has some 1,800 fighters, including non-Syrians, according to the Observatory.
The group and its ally Ansar Al Tawhid both operate in the Idlib region and are members of a joint militant operation room that also includes Al Qaida's former Syria affiliate, Hayat Tahrir Al Sham (HTS).
Most of Idlib province and parts of neighbouring Aleppo and Latakia provinces are controlled by HTS.
Syrian state news agency SANA on Saturday said the government agreed to the Idlib ceasefire deal, which Russia said aimed "to stabilise the situation" in the anti-government bastion.
The Idlib region is home to some three million people, nearly half of whom have been displaced from other parts of Syria.
The Syrian conflict has killed more than 370,000 people and driven millions from their homes since it started with the brutal repression of anti-government protests in 2011.