The Observatory said the bodies of the IS militants were still tossed on the streets of Ayn al-Arab, adding that three explosions rocked that city, when three IS suicide bombers detonated three car bombs in different areas, mainly on the border point between Ayn al-Arab and Turkey.

The IS surprise attack against Ayn al-Arab has sent the chill down the civilians' spines there, as the IS had overran the city before and was forcibly dislodged out of the city five months ago.

Upon entering the town of Barkh Butan in southern Ayn al-Arab, the IS assailants shot dead 20 Kurdish civilians, who held up weapons to defend their town.

Observers said the IS wanted to storm Ayn al-Arab to revenge the losses it has recently suffered at the hands of Kurdish fighters in predominantly Kurdish areas in northern Syria.

Over the past few days, the YPG and allied rebel groups stripped the IS of several towns and areas in the northern countryside of the IS de facto capital of Raqqa in northern Syria,

pushing the IS to call for back up forces to fortify its positions in the city of Raqqa.

Reports said the YPG and affiliated rebel groups are only 30 km from central Raqqa.

In another major offensive Thursday, the IS militants launched a wide-scale assault on Syria's northern city of Hasaka, storming districts in the southern part of that city, whose northern and northwestern parts contain a large community of Kurds.

Intense clashes broke out between Syrian government forces and IS militants in the southern districts of Hasaka, according to the Observatory.

The London-based watchdog said the battles were coupled with intense shelling by both sides, and government airstrikes against the positions overran by IS in southern Hasaka.

IS militants detonated at least one booby-trapped vehicle at a government troops' checkpoint at the western entrance of Hasaka, said the Observatory, adding that the militants reached the Sport City street, adjacent to the Central Prison and the Criminal Intelligence Branch, after storming districts of Nashweh and Sharia in the southern and southwestern parts of Hasaka.

According to the Observatory, which relies on a network of activists on the ground inside Syria, at least 30 government soldiers and 20 IS militants were killed during Thursday's battles.

Meanwhile, Syria's state-run al-Ekhbarieh TV said government troops are fighting intense battles against IS militants in Nashweh neighborhood, which, it said, houses many displaced families from the eastern province of Deir al-Zour, where the IS holds considerable sway.

It said the IS are committing crimes against the residents of that neighborhood.