The United States military has said it hit an al-Qaeda-linked training camp in northwest Syria's rebel-held Idlib province.
The US Central Command (CENTCOM) said in a statement on Saturday its attack close to Idlib city targeted leaders that were "responsible for attacks threatening US citizens, partners, and innocent civilians."
"The removal of the facility will further degrade [al-Qaeda's] ability to conduct future attacks and destabilize the region," the statement added, without mentioning what kind of weaponry was used.
A war monitor said "at least 40" fighters were killed in what it called a missile attack. CENTCOM declined to say what kind of weaponry was used in the attack in Idlib, the last remaining bastion for anti-government rebels in Syria that has been the target of a Russia-backed government offensive since April.
The air attack "targeted a meeting held by the leaders of Hurras al-Deen, Ansar al-Tawhid and other allied groups inside a training camp," the Britain-based Syrian Observatory for Human Rights said on Saturday.
Al Jazeera's Bernard Smith, reporting from Hatay, near the Syria-Turkey border, quoted sources on the ground as saying that the area targeted was "a training centre connected with al-Qaeda".
"[The training centre] encouraged people from all ages - young boys to teenage men to older men - to come there for training," Smith said, adding that videos posted after the attack showed at least one wounded child.
On July 1, the US said it had carried out a raid on the al-Qaeda-linked Hurras al-Deen armed group in northwest 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 operation room that also includes al-Qaeda's former Syria affiliate, Hay'et Tahrir al-Sham (HTS).
Most of Idlib province and parts of neighbouring Aleppo and Latakia provinces are controlled by HTS.
The Russian military on Sunday said the US carried out an air raid in the Idlib de-escalation zone in breach of previous agreements, inflicting numerous casualties and endangering a ceasefire, according to news agencies in Russia,
TASS news agency, citing the Russian defence ministry, said the United States had forewarned neither Russia nor Turkey about the air raids.
Russia, which supports Syrian President Bashar al-Assad, and Turkey, long a backer of the rebels, co-sponsored a de-escalation agreement for the area that has been in place since last year but faltered in recent months, forcing hundreds of thousands of civilians to flee.
Separately, Syrian government air raids on the Idlib region halted early on Saturday, after the government agreed to a Moscow-backed ceasefire following four months of deadly bombardment.
Russia-backed government forces have been pressing an offensive against the major opposition stronghold in Idlib since late April.
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.
But the army "reserves the right to respond to violations" by 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 United Nations says the violence has also displaced more than 400,000 people.
The region is supposed to be protected from a significant government offensive by a Turkey-Russia deal struck in September 2018 that was never fully implemented.
Turkey backs various rebel groups in northwest Syria.
"Russia and the Syrian government may be willing to give Turkey another opportunity to implement the terms of its September 2018 bilateral agreement with Russia," said Sam Heller of the International Crisis Group.
"Alternately, this ceasefire may just be an operational pause for Damascus and Moscow to consolidate their territorial gains and prepare for the next phase of their offensive," he told AFP news agency.
Syrian President Bashar al-Assad, whose forces control about 60 percent of the territory, has promised to reclaim the rest of the country, including Idlib.
Saturday's truce is the latest attempt to avert a full-blown offensive, which the UN has said would result in one of the worst humanitarian "nightmares" in Syria's eight-year conflict.
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.