The Damascus government says Turkish forces have entered war-torn northern Syria to help foreign-backed militants fighting against the government of President Bashar al-Assad.

The Syrian Foreign Ministry in a protest letter to the United Nations Security Council (UNSC) said on Sunday that “around 100 gunmen some of whom are believed to be Turkish forces and Turkish mercenaries" entered Syria through A'zaz area which is located north of Aleppo near the Turkish border.

It also noted that the Turkish forces were accompanied by a dozen pick-up trucks mounted with heavy machine guns.

"The operation of supplying ammunition and weapons is continuing via the Bab al-Salama crossing to the Syrian area of Azaz," the ministry said.

On Saturday, the Turkish army bombed the A’zaz border crossing near an airbase in Syria's northern province of Aleppo for the second consecutive day. The attack was launched days after Syrian Kurdish fighters seized the area from Daesh terrorists.

The move came a few days after the Kurdish fighters, backed by Russian air raids, drove foreign-backed militants from the Menagh air base near the Turkish frontier.

Ankara considers the Kurdish People’s Protection Units (YPG) to be the Syrian branch of the outlawed Kurdistan Workers’ Party ( PKK) that has been fighting for an autonomous Kurdish region inside Turkey since the 1980s.

The YPG, which controls nearly all of Syria’s northern border with Turkey, has been fighting against Daesh Takfiri terrorist group.

Kurdish female fighters of the People's Protection Units (YPG) load their weapons during a training at their camp in the city of al-Qahtaniyah, northeastern Syria, on December 1, 2015. (AFP photo)

Meanwhile, France has urged Turkey to stop shelling the positions of Kurdish fighters in northern Syria.

A French foreign ministry statement said Ankara's decision could deteriorate an already volatile situation in Syria’s north. The statement called on Turkey to join the ceasefire efforts instead.

The United States has also denounced Turkey's decision as untimely.

The developments came as Turkish Foreign Minister Mevlut Cavusoglu said Saudi Arabia and his country could launch a ground operation in Syria.

Turkey has been among the main supporters of the militant groups operating in Syria, with reports saying that Ankara actively trains and arms the Takfiri terrorists there and facilitates their safe passage into the conflict-ridden Arab country.

Turkey has also been accused on numerous occasions of being involved in illegal oil trade with Daesh. Russia has released pictures and videos purportedly showing the movement of oil tankers from Daesh-controlled areas in Syria toward Turkey.

The foreign-sponsored conflict in Syria, which flared in March 2011, has claimed the lives of some 470,000 people and left 1.9 million injured, according to the Syrian Center for Policy Research.