The Syrian government says Turkish forces were believed to be among 100 gunmen it said entered Syria on Saturday accompanied by 12 pick-up trucks mounted with heavy machine guns, in an ongoing supply operation to insurgents fighting Damascus.

"The operation of supplying ammunition and weapons is continuing via the Bab al-Salama crossing to the Syrian area of Azaz," the Syrian foreign ministry said in a letter to the UN Security Council published by state news agency SANA.

It said the pick-up trucks were "accompanied by around 100 gunmen some of whom are believed to be Turkish forces and Turkish mercenaries".

The same letter also criticised Turkey for shelling areas of northern Syria. Azaz is north of Aleppo near the Turkish border.