Russia and Turkey have troops on the ground in Idlib and the US did not give notice of the strike.
Idlib and other adjacent territories of Syria held by jihadists is facing intensified bombardment in the past month.
Rebels who have fought to topple President Bashar al-Assad for 8 years are now largely confined to the enclave in the northwest near the Turkish border.
Idlib, the last major part of Syria still outside the control of the regime of President Bashar al-Assad, is held by an alliance led by Syria's former Al-Qaeda affiliate.
Fighting erupted when Syrian troops seized a position in a rural area in the north of neighbouring Hama province that had been held by the Jaish al-Izza group.
The main jihadist group in the northwest, Tahrir al-Sham, gave a nod of approval to the Turkish agreement, but without explicitly saying it would abide by it.
Three children were among those killed in the country's last major rebel bastion in the northwest of the country.
Their withdrawal was seen as the real test of the accord reached on September 17 between rebel backer Ankara and regime supporter Moscow in the Russian resort town of Sochi.
Syria's war has killed more than 360,000 people since starting in 2011 with the brutal repression of anti-government protests.
All rebels in the demilitarised zone must withdraw heavy arms by Wednesday, and radical groups must leave by October 15.
The group is the first to comply with a requirement to leave a demilitarised buffer zone.
SARAJEVO: Russia and Turkey have agreed on borders of the demilitarised zone around Syria's Idlib, Russia's top diplomat said Friday, part of a deal...
Russian President Vladimir Putin and his Turkish counterpart Recep Tayyip Erdogan on Sept 17 met in the Russian Black Sea resort of Sochi, and agreed to create a demilitarised buffer zone between rebels and regime troops by October 15.
The incident, which happened late Monday, was the worst case of friendly fire between the two allies since Russia's game-changing military intervention in September 2015.
Turkey, which supports rebels opposed to Syrian President Bashar al-Assad, has been holding talks with his allies Russia and Iran over the fate of Idlib and surrounding districts which Assad has vowed to recapture.
Turkey has intensified negotiations with Russia to avert a possible attack, repeatedly calling for a ceasefire.
Russia-backed regime forces have massed around Idlib in recent weeks, sparking fears of an imminent air and ground attack to retake the last major opposition bastion.
Syrian government and Russian warplanes began air strikes in Idlib last week in a possible prelude to a full-scale offensive and aid organisations said several medical facilities have already been targeted.
About half of those displaced so far have moved to camps, while others went to informal settlements, stayed with families or rented housing.
During the seven-year Syrian conflict, the West has accused Damascus on a number of occasions of using chemical weapons against the civilian population.
Erdogan has called for a ceasefire in the northwestern province of Idlib, the last rebel stronghold in Syria, as an assault by Syrian government forces is expected any day.
A major military operation in Idlib is expected to pose a humanitarian nightmare because there is no nearby opposition territory left in Syria where people could be evacuated to.
Many travelled to Syria as early as 2012, marrying Syrian women and starting families there, so they are likely to do all they can to protect their new home.
Aid organisations have warned that any military campaign to retake the region could spark one of the worst humanitarian disasters in Syria's seven-year war.
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