The US says it has carried out an attack on leaders of a group it calls al-Qaeda in Syria, in the country's rebel-held Idlib province.
US Central Command said the operation had targeted those "responsible for attacks threatening US citizens, our partners and innocent civilians".
No details were given but other reports say some 40 people died in a missile strike on a jihadist training camp.
It was hit just after Syrian government forces began a truce in Idlib.
Initial reports indicated that calm had settled on the front lines after the Russian-backed unilateral ceasefire, which started at 06:00 (03:00 GMT).
In a separate development, Turkey warned it would launch an operation to set up a "safe zone" in north-eastern Syria if talks with the US on the issue failed.
"Within a few weeks if our soldiers do not start to actually control this area, there will be no other option left but to implement our own operation plans," Turkish President Recep Tayyip Erdogan said.
Ankara and Washington are at odds over the future of the region where the US-backed Kurdish YPG militia has been fighting so-called Islamic State.
Turkey considers the YPG a terrorist group.
Earlier this month, the Kurdish-led authorities in north-eastern Syria reportedly began withdrawing from territory along the border with Turkey.
What is happening in Idlib?
Forces loyal to the government launched an aerial offensive aimed at re-taking the area late in April and ground forces have also been making advances in recent weeks.
They have retaken key strategic locations, such as the town of Khan Sheikhoun - which had been held by rebels for five years.
The UN has issued repeated warnings that the offensive is causing a humanitarian disaster - within a civil war that has already taken hundreds of thousands of lives and displaced millions.
The war in Syria: five questions answered
How did the war start?
The country descended into war after President Bashar al-Assad's government used deadly force to crackdown on pro-democracy protesters, who took to the streets in March 2011 demanding political freedom.
Who is fighting?
That's complicated: President Assad's regime is fighting rebel groups ranging from pro-democracy groups to jihadist extremists, while a number of foreign powers are providing support to various sides.
How many people have died?
It is not known exactly, as death tolls vary according to the source, but it is estimated to stand at more than 500,000 dead or missing.
How many refugees are there?
More than 5.6 million people have fled Syria since 2011, with another 6.6 million internally displaced, according to the UN.
What has happened to President Assad?
His position looked tenuous at one point during the eight-year conflict, but thanks to international allies like Russia and Iran, President Assad has won back control of most of Syria, and has set his sights on Idlib.