A US drone strike against Al Qaeda in north-western Syria killed 17 members of the terrorist group including 11 leaders, a war monitor said.
Five civilians were also among those killed in the strike near the Turkish border, the Syrian Observatory for Human Rights said.
"US Forces conducted a strike against a group of Al Qaeda in Syria (AQ-S) senior leaders meeting near Idlib, Syria," said Major Beth Riordan, the spokeswoman for United States Central Command (Centcom).
"The removal of these AQ-S leaders will disrupt the terrorist organisation's ability to further plot and carry out global attacks threatening US citizens, our partners and innocent civilians," Major Riordan said in a statement.
She did not specify the number of deaths from the strike.
But the Britain-based Observatory said the operation, which targeted a dinner meeting in the village of Jakara in the Salqin area, killed at least 17 militants including 11 leaders.
The village lies in Syria's last major rebel bastion of Idlib, which is dominated by the Hayat Tahrir Al Sham (HTS) group, led by a former Al Qaeda affiliate, and its rebel allies.
But other extremist groups, including the rival Al Qaeda-linked Hurras Al Deen faction, are also present in the area.
Observatory chief Rami Abdel Rahman said that five non-Syrian extremists were among those killed, but their nationalities were not immediately known.
"They had been invited to dinner in a tent on a farm in Jakara," he said.
"It was a meeting of leaders opposed to HTS and who reject the Russia-Turkish deals" that led to a fragile truce in Idlib, he said. "Some were close to Hurras Al Deen."
A March agreement between rebel backer Turkey and government ally Russia halted a deadly offensive by government forces against the Idlib region of some three million people.
The US military has a presence in the east of Syria, where its air strikes have backed Kurdish-led forces battling remnants of ISIS.
Thursday's strike came after it emerged that the 18-year-old who killed a school teacher in France last week for showing cartoons of the Prophet Mohammed in class had been in contact with a Russian-speaking extremist in Syria.
A source close to the case said the identity of the contact was not yet known, but French newspaper Le Parisien claimed the person's IP address was traced to Idlib.
After a string of military victories backed by Russia, the Syrian government has regained control of around 70 per cent of the country, with the Idlib region the last major territory held by rebels.
Syria's civil war, which broke out after the bloody suppression of anti-government protests in 2011, has killed more than 380,000 people and displaced millions from their homes.
