BEIRUT — A suspected Syrian government airstrike on a market in a rebel-held town in northwest Syria killed 13 civilians on Monday, while Turkish artillery shells landed near a school in a Kurdish-held town, killing at least nine people, including eight children, activists said.

The violence is contributing to rising tension in Syria’s north, along the border with Turkey. Syrian government troops have renewed a push against the last opposition stronghold, which is in Idlib Province. And Turkey, which views Syrian Kurdish fighters as an existential threat, has been expanding its military operations there to push the Kurdish fighters farther from the border.

The Syrian Observatory for Human Rights, based in Britain, said the Syrian airstrike hit a popular market in the rebel-held town of Maaret al-Numan, killing 13 people and injuring 18. The opposition-run Aleppo Media Center also put the death toll at 13, although the Syrian Civil Defense, a team of emergency medical workers known as the White Helmets, said that nine civilians, including two women, had been killed.

Different casualty tolls are common in the chaos of the civil war. Over the weekend, fighting between Syrian government forces and insurgents in Idlib resulted in dozens of deaths on both sides.