Shelling of the rebel-held city of Afrin in northern Syria killed at least 18 people on Saturday, many of them when a hospital was hit, a war monitor said.

The Syrian Observatory for Human Rights said a doctor, three hospital staff, two women and two children died at Al-Shifaa hospital in the city which is held by Turkish-backed rebels.

A rebel commander also died at the hospital, the Observatory said, adding that 23 people were injured.

An AFP correspondent shot footage of white-helmeted aid workers in the hospital courtyard strewn with bodies.

"The shelling targeted several areas of the town and hit the hospital", Observatory director Rami Abdel Rahmane told AFP.

"Most of the victims died in shelling on the hospital," the monitoring group said in a statement, warning the casualty toll could rise further with some of the wounded in a critical condition.

The artillery fire originated from northern Aleppo province "where militia faithful to Iran and the (Syrian) regime are deployed, near the zones run by Kurdish forces", the Britain-based group said.

The Kurdish-led Syrian Democratic Forces (SDF) issued a statement denying any involvement in the shelling.

The region, like all areas held by pro-Turkish rebels, regularly witnesses targeted killings, bombings and shootings.

Syrian regime shelling on the opposition-held Idlib enclave killed 12 people Thursday, one of the deadliest violations of a 15-month-old ceasefire, the war monitor said.

The conflict in Syria has killed nearly 500,000 people since it started in 2011 with the brutal repression of peaceful demonstrations.