Syrian regime forces shelled the town of Ariha in the northwestern province of Idlib on Sunday killing at least 13 people, a monitoring group said.

"Thirteen civilians at least were killed in shelling by regime forces on the centre of the town of Ariha in Idlib province," Syrian Observatory for Human Rights head Rami Abdel Rahman told AFP.

Dozens were also wounded in the shelling which struck the main market in the centre of the town.

Regime forces opened fire from positions they control on the edges of the town, Abdel Rahman said.

It was not immediately clear what triggered the shelling, but rebels control part of Ariha while regime forces man three checkpoints inside the town.

The Observatory said clashes had erupted in Ariha over the week, as rebels try to expand their control over the town.

Activists termed the latest bloodshed a "massacre" and posted a video online showing people carrying corpses and body parts covered in blood as cries were heard in the background.