U.S. forces attacked jihadist leaders in northwestern Syria on Saturday, the Pentagon said, in a missile strike that left at least 40 dead, according to a Britain-based monitor.

The U.S. strike came as renewed bombardment of Idlib province by government forces killed one civilian in a first violation of a Russian-backed truce for the region that came into effect just hours before, the Syrian Observatory for Human Rights said.

The U.S. missile attack "targeted a meeting held by the leaders of Hurras al-Deen, Ansar al-Tawhid and other allied groups inside a training camp", said Rami Abdel Rahman, head of the Observatory. The monitor said at least 40 jihadist leaders were killed.

The U.S. Central Command said in a statement that the attack targeted leaders of Al-Qaeda in Syria (AQ-S) "responsible for attacks threatening US citizens, our partners and innocent civilians."

CENTCOM did not say what kind of weaponry was used. It was not immediately clear if the missiles were launched from war planes or positions on the ground, the monitor said.

On July 1 the U.S. said it had carried out a strike on Hurras al-Deen, an Al-Qaeda-linked group established in February 2018 with some 1,800 fighters in northwestern Syria, in its first such operation there in two years.

The group and its ally Ansar al-Tawhid both operate in the Idlib region and are members of a joint jihadist operation room that also includes Al-Qaeda's former Syria affiliate, Hayat Tahrir al-Sham (HTS), which controls most of Idlib and parts of neighboring Aleppo and Latakia provinces.

Syrian government air strikes on the jihadist-run Idlib region had halted early on Saturday, after the government agreed to a Moscow-backed ceasefire following four months of deadly bombardment, the monitor said.

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

Russia-backed government forces have been pressing an offensive against the major opposition stronghold in Idlib since April.

Syrian state news agency SANA reported Saturday morning that Damascus agreed to the Idlib ceasefire deal, which Russia said aimed "to stabilize the situation" in the anti-government bastion.

But the army "reserves the right to respond to violations" by jihadists and allied rebel groups, SANA added, citing a Syrian military source.

The Idlib region is home to some three million people, nearly half of whom have been displaced from other parts of Syria. The region is supposed to be protected from a massive government offensive by a Turkish-Russian deal struck in September 2018 that was never fully implemented as jihadists refused to withdraw from a planned demilitarized cordon.

Assad, whose forces control around 60 percent of territory, has vowed to reclaim the rest of the country, including Idlib.

The UN has said a full-blown offensive against the Idlib region would result in one of the worst humanitarian "nightmares" in Syria's eight-year war.