"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 Abdel Rahman, head of the Syrian Observatory for Human Rights.

The attack killed "at least 40" jihadist leaders, the Britain-based monitor said.

It was not immediately clear who was behind the attack, or if the missiles were launched from war planes or positions on the ground, the Observatory said.

An AFP correspondent saw clouds of black smoke rising over the area after blasts rocked the jihadist stronghold.

Ambulances rushed to the site of the attack, which was closed off to journalists, he said.

The Al-Qaeda-linked Hurras al-Deen and their allies Ansar al-Tawhid both operate in the Idlib region, where bombardment by Damascus and Moscow came to a halt Saturday morning after the government agreed to a Russian-backed ceasefire following four months of deadly bombardment.

Both groups are members of a joint jihadist operation room that also includes Al-Qaeda's former Syria affiliate, Hayat Tahrir al-Sham.

Hurras al-Deen was established in February 2018 and has some 1,800 fighters, including non-Syrians, according to the Observatory.

On July 1 the United States claimed it had carried out a strike on Hurras al-Deen in northwestern Syria, in its first such operation there in two years.

"US forces conducted a strike against Al-Qaida in Syria (AQ-S) leadership at a training facility," US Central Command said in a statement at the time.

The Observatory had reported eight members of the Al-Qaeda-linked group, including six commanders, were killed in a June 30 attack.

The United States had previously carried out several strikes in northwestern Syria, but they appeared to have petered out since 2017.