The attack on the rebel-held town of Ariha, which took place shortly after the Damascus bombing, had caused the biggest civilian death toll in the Idlib area since March, 2020, the Syrian Observatory for Human Rights said.
A decade of conflict in Syria has killed hundreds of thousands of people and fractured the country.
There was no immediate claim of responsibility for the Damascus bombing, which hit a bus carrying army personnel in the middle of the city at around 6:45 a.m. (0345 GMT), state television reported.
A military source quoted by state media said the bus was blown up by two bombs that had been attached to the vehicle in advance. A third device was defused by an army engineering unit.
Syrian state TV posted on its Telegram account images of the charred cabin of the bus, and rescue workers could be seen removing body parts.
The attack happened as the bus was passing near a bridge named after President Hafez al-Assad, who died in 2000, the father of current President Bashar al-Assad.
“We will pursue the terrorists who committed this heinous crime wherever they are,” Interior Minister Mohamad al-Rahman said in comments reported by state media.
Dozens of people were killed in Damascus in 2017 in several suicide attacks claimed by jihadist groups, including two against police stations which the Islamic State group claimed to have carried out.
Attacks in Damascus have been rare since the army crushed rebel enclaves around the city with backing from Russia and Iran-backed forces in 2018. Helped by his allies, Assad now controls most of the country.
Islamic State militants still operate in the deserts of central and eastern Syrian desert, where they have mounted several attacks this year on army vehicles.
Northwestern Syria is the last major stronghold of rebels fighting Assad. The witnesses and rescue workers said shelling struck residential areas of the rebel-held town of Ariha shortly after the Damascus bomb attack.
Among the casualties were several school children, witnesses and medical workers in the opposition enclave said. Thirty people were wounded.
Turkey’s state-run Anadolu Agency said government forces and Iran-backed groups targeted a market place in the Ariha town centre.
After struggling to make any headway, United Nations-backed efforts to reach a political settlement to the war took a step forward on Sunday when the U.N. envoy for Syria said the government and opposition had agreed to draft a new constitution.
(Reporting by Suleiman al-Khalidi in Amman and Jonathan Spicer in Turkey; Writing by Maher Chmaytelli/Tom Perry; Editing by Angus MacSwan)