"17 martyrs joined yesterday the convoy of martyrs of the Syrian revolution," said Syrian Observatory for Human Rights.

The group announced that 13 people including 7 children and 3 women, were killed in a horrific massacre by regime's warplanes and helicopters which targeted Mahmbel village in the western countryside of Idlib.

"3 children were killed in bombardment by the warplanes, and the rest of them were killed in a bombing by helicopters."

The observatory also said a woman was killed in artillery shelling by the Kurdish Forces targeted the western neighborhood of Marea city in the northern countryside of Aleppo.

The Syrian Observatory also documented the death of 3 children in the explosion of a landmine on the outskirts of Jobar neighborhood east of the capital Damascus.

As the last stronghold of oppositions against the regime, Idlib has been the focus of a regime and Russian offensive since late April. 

The province, which borders Turkey, was supposed to be protected by a deal struck between Turkey and Russia but has instead been subjected to heavy bombardment during the ongoing campaign. 

An average of two children has been killed in Idlib every day, according to the estimate of another activist group, the Syrian Network for Human Rights. 

The group said 518 people were killed in a two-month period from the end of April.

The United Nations says 25 health facilities in the region have been hit, the latest including the second attack in two months on an underground hospital in the town of Kafranbel on Thursday.

"The attacks happened despite the fact that the coordinates of this hospital had previously been shared with the parties to the conflict in a deliberate, carefully planned effort to prevent any attacks on it," a UN official said on Friday.

"I am horrified by the ongoing attacks on civilian areas and civilian infrastructure as the conflict in northwest Syria continues," said Mark Cutts, UN deputy regional humanitarian coordinator for the Syrian crisis.

Syria's war has killed more than 370,000 people and displaced millions since it started in 2011 with a brutal crackdown on anti-regime protests. (ILKHA)