A US military strike reportedly killed dozens of militant leaders in Syria's Idlib province, hours after a ceasefire went into effect on Saturday.
The US Central Command said the attack was carried out against "Al Qaeda in Syria leadership at a facility north of Idlib".
The leaders were "responsible for attacks threatening US citizens, our partners and innocent civilians", the centre said.
The Syrian Observatory for Human Rights, an opposition war monitor, said a missile struck a meeting of militant leaders at a training camp just north of Idlib city, killing 40 of them.
The leaders of Hurras Al Din, Ansar Al Tawhid and other allied groups were present, said Rami Abdulrahman, head of the Observatory.
Russia, an ally of Syrian President Bashar Al Assad, accused the US of jeopardising the ceasefire with its strike.
The US had given no advance warning to Russia or Turkey, the Tass news agency said.
Mr Al Assad's forces, backed by Russia, have been bombing Idlib province and adjoining areas in north-western Syria since April in to recapture the last major territory held by opposition fighters after more than eight years of civil war.
The US has troops in north-east Syria to support allied Syrian militias in the battle against ISIS, but operations in the north-west region are rare.
On July 1 the US said it had carried out a strike on Hurras Al Din in north-western Syria, in its first such operation there in two years.
Hurras Al Din, linked to Al Qaeda, was established in February 2018 and has about 1,800 fighters, including non-Syrians, the Observatory said from Britain.
The group and its ally, Ansar Al Tawhid, operate in the Idlib region and are parts of an extremist coalition that includes Al Qaeda's former Syria affiliate, Hayat Tahrir Al Sham.
Most of Idlib province and parts of neighbouring Aleppo and Latakia are controlled by Hayat Tahrir Al Sham.
Syrian government air strikes on the Idlib region halted early on Saturday, after the regime agreed to a Moscow-backed ceasefire.
It was the second time the government has suspended its bombardment of the region after a truce declared on August 1 lasted only a few days.
But regime bombardment on the town of Kafranbel late on Saturday killed a civilian, the Observatory said.
The army "reserves the right to respond to violations" by extremists and allied rebel groups, Syria's state news agency said.
The latest truce came after mounting international pressure on Syria's allies over the high civilian death toll and repeated attacks on non-mliitary targets such as hospitals, markets and schools.
A draft UN Security Council resolution put forward by Germany, Kuwait and Belgium expresses "outrage at the unacceptable levels of violence escalating in and around Idlib province”.
Syrian and Russian air strikes have killed more than 950 civilians since the end of April, according to the Observatory. The UN says the violence has also displaced more than 400,000 people.
Turkey fears that Idlib residents, many of whom were already displaced by fighting in other parts of Syria, will flood across the border if the offensive continues, and join the millions of Syrians who have sought refuge in the country since 2011.
Ankara and Moscow brokered a truce for the region in September last year to prevent a massive government offensive but it was never fully implemented as extremists refused to withdraw from a planned demilitarised cordon.
Turkey backs Syrian rebels groups in north and north-west 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," Sam Heller, of the International Crisis Group, told AFP.
"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."
Mr Al Assad, whose forces now control about 60 per cent of Syrian territory, has vowed to reclaim the rest of the country, including Idlib and the Kurdish-controlled north-east.
The Syrian conflict is estimated to have killed more than 400,000 people since 2011 and has driven millions into neighbouring Lebanon, Jordan and Turkey.