Syrian Kurdish militants from the People's Protection Units (YPG) have captured a strategically important air base from the grip of ISIS in the Raqqa province, northern Syria, according to a spokesman for the Kurdish forces and activists.
Reuters quoted a spokesman for the Kurdish forces saying the troops seized control of the "Brigade 93" air base in Raqqa.
The Syrian Observatory for Human Rights (SOHR) also said Kurdish forces "fully controlled" the "Brigade 93" air base, after forcing ISIS to retreat deeper into Raqqa.
Raqqa is the main stronghold of ISIS in Syria, and is the de facto capital of their self-proclaimed “caliphate” which they established in areas of Syria and Iraq under their control.
"They have been defeated," said Redor Xelil, a Kurdish general, while referring to ISIS, which seized the base from the Syrian regime forces last year.
Kurdish fighters approached the town of Ain Issa, which is located 50 kilometres to the north of the city of Raqqa.
US-led coalition air strikes are supporting Kurdish forces fighting to capture areas under ISIS control in the region.
"This is an important base for ISIS because it overlooks the roads linking its stronghold in Raqqa, Idlib and Hasaka," said Rami Abdul Rahman, the director of the Observatory.
"This means the continuation of cracking inside the stronghold of ISIS."
Kurdish forces have also taken control of the Syrian city of Tal Abyad, on the border with Turkey. This development has been described as a strong blow to the militant organisation in the Middle East.
Tal Abyad was a major artery through which the foreign fighters flow supplies the territory controlled by ISIS in Syria.
An official at the Syrian opposition National Coalition, based in Turkey, told the BBC on Monday that the Kurdish People's Protection forces refused to allow a delegation from the Commission of Inquiry set up by the coalition to enter the city of Tal Abyad.
The aim of the delegation is to oversee the situation of the fleeing Syrian locals and listen to their testimony on the events that followed the Kurdish fighters battle with ISIS in the city last week.