At least 30 people were killed and dozens injured when a Syrian army fighter jet crashed into a busy marketplace in the rebel-held northwestern town of Ariha.
Most of the dead were civilians on the ground in the Idlib provincial town that fell to a coalition of Islamist insurgents in May.
Scores were also injured, according to witnesses. There was no immediate reaction from the Syrian army.
The military plane had dropped a bomb in the heart of the city centre main commercial street, where shopkeepers open in the early morning, before crashing in the middle of the marketplace, two witnesses told Reuters news agency.
"The plane had dropped a bomb on the main Bazaar street at low altitude only seconds before it crashed," said Ghazal Abdullah, a resident who was close to the incident.
The Observatory said the jet was not shot down.
Fighting has intensified of late in rural Idlib province between government forces and an insurgent grouping called Jaishal Fateh, or Army of Conquest, which includes Syria's al-Qaeda offshoot Nusra Front.
Ariha's fall had left the insurgents in control of most of Idlib province, which borders Turkey and neighbours Latakia, the heartland of President Bashar al-Assad's Alawite sect, on the Mediterranean coast.
Most of the rich agricultural region, however, has since come under heavy aerial bombardment by Mr Assad's forces in a counter-offensive to regain lost ground.
The army has fought back using heavy air strikes to beat back insurgent advances into the mountains of Latakia province that brought them closer to government-held coastal areas north of the capital Damascus.
Syria's western flank, fringing both the Mediterranean coast and the Lebanese border, contains Syria's major cities including Damascus and is seen as crucial for Mr Assad's hold on power.