Idlib (Syria) (AFP) - US forces attacked jihadist leaders in northwestern Syria on Saturday, the Pentagon said, in what a battlefield monitor called a missile strike that left at least 40 dead.

The US strike came as renewed Syrian regime bombardment of Idlib province 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 US Defense Department said its own attack targeted leaders of Al-Qaeda in the north of the same province, but did not say what kind of weapon was used.

That attack targeted leaders of jihadist groups and allied factions near Idlib city, the Observatory said.

The US 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.

It killed at least 40 jihadist leaders, the Britain based monitor said.

The US 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.

"Additionally, the removal of this facility will further degrade their ability to conduct future attacks and destabilize the region."

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.

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

CENTCOM declined to say what kind of weaponry was used.

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.

Al-Qaeda-linked 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 jihadist operation room that also includes Al-Qaeda's former Syria affiliate, Hayat Tahrir al-Sham (HTS).

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Most of Idlib province and parts of neighbouring Aleppo and Latakia provinces are controlled by HTS.

- 'Temporary' truce -

Syrian government air strikes on the jihadist-run Idlib region had halted early on Saturday, after the regime 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 regime and jihadists since August 1.

But regime bombardment on the town of Kafranbel late in the day left one civilian dead, just hours after the truce came into effect, the monitor said.

Hours later, two regime loyalists were reported killed when their car was targeted by rebel and jihadist fighters along Idlib's southern border with Hama, according to the Observatory.

It also said HTS fighters had downed a Russian reconnaissance drone.

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

On Saturday, Assad advisor Buthaina Shaaban said in an interview that the ceasefire was "temporary".

It "serves the grand strategy of liberating every inch of Syrian territory", she told Lebanon's pro-Damascus Al-Mayadeen TV.

- 'Nightmares' -

Syrian state news agency SANA reported Saturday morning that Damascus agreed to the Idlib ceasefire deal, which Russia said aimed "to stabilise 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.

Air strikes by Damascus and Russia have killed more than 950 civilians since the end of April, according to the Observatory.

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

The Idlib 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 demilitarised cordon.

Turkey backs rebels in northwestern Syria.

Assad, whose forces control around 60 per cent 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.

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.