This record is one of the deadliest inflicted on the jihadists in a single attack in this war-torn country.
"This operation was targeting AQ-S leaders responsible for attacks threatening US citizens, our partners, as well as innocent civilians," the Pentagon said in a brief statement, without giving details of how the operation had been conducted.
The Syrian Observatory for Human Rights (OSDH), which draws on a vast network of sources in Syria, said that missile fire had targeted a meeting gathering leaders jihadist groups Hourras al-Din and Ansar al-Tawhid and leaders of other extremist groups who are allied with them in a training camp.
"At least 40 of them" were killed, told AFP Rami Abdel Rahman, director of the OSDH.
The attack came on the first day of a truce in the bombardment of the Syrian regime and its Russian ally against the Idleb region.
The targeted groups are allies of the HTS, the former Syrian branch of Al Qaeda. All these factions have already been the target of air raids by the Syrian regime, its Russian ally, but also the international anti-jihadist coalition led by the United States, and the United States itself.
North of Idleb, successive explosions were heard, followed by columns of smoke, said an AFP correspondent present near the scene of the attack. Ambulances went on site while journalists did not have access.
On June 30, the United States conducted a strike "against the leadership of Al Qaeda in Syria in a training facility" in the province of Aleppo, next to that of Idleb. The OSDH then claimed that the strike had killed eight, including six commanders of the group Hourras al-Din.
In 2014, the United States set up an international coalition to fight the Islamic State (IS) jihadist group, which was defeated last March in Syria with the help of Kurdish forces. But American soldiers are still there in Syria. US strikes against jihadists have dropped significantly since 2017.