The strikes on Darkush came a day after rebels seized Jisr al-Shughur, another town some 20 kilometres (12 miles) to the south and in the same province of Idlib.
“At least 40 civilians were killed in regime air strikes on a market in the town of Darkush. Among the dead were nine women and eight children,” said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
He had said earlier that an initial toll of 34 dead had been expected to rise “because there are dozens of people injured and many in serious condition”.
Jisr al-Shughur was one of the regime’s last remaining strongholds in the northwestern province, and its fall has left government-held territory elsewhere in neighbouring Latakia and Hama provinces open to new attack.
Earlier on Sunday, regime warplanes carried out dozens of strikes on Jisr al-Shughur, which was captured by a coalition of groups including Al-Qaeda’s Syrian affiliate Al-Nusra Front.
The same coalition captured the city of Idlib last month, making it only the second provincial capital to fall from government control.
The Britain-based Observatory also reported fierce fighting in the northeastern province of Hasakeh between Kurdish People’s Protection Units (YPG) and Islamic State group jihadists.
It said 16 IS militants and two Kurds were killed near Tal Tamr close to Syria’s border with Turkey.
Abdel Rahman said the YPG fought off an attack on Tal Tamr that the jihadists have been trying to take for some time because of its strategic location.