ISIS has called on its militants to strengthen the defense of the city of Raqqa, the main stronghold of the group in northern Syria, Syrian opposition sources have said.
Kurdish People's Protection Units, or YPG, have said they have inflicted large losses on ISIS, and most recently expelled them from the “Brigade 93” air base, a Syrian military facility that was formerly under the control of the Assad regime and which ISIS controlled for a full year.
The YPG forces currently gained full control over the town of Ain Issa near Raqqa.
BBC reports say that the Kurdish troops are about 50 kilometres away from the city of Raqqa, the self-proclaimed capital of ISIS’ "caliphate."
The BBC correspondent in Damascus said “ISIS is enhancing defensive lines by transferring militants from Der el Zor and Hasaka to Raqqa."
The United States considers the developments in the area to be a "model" for coordination between the international coalition forces and anti-ISIS fighting forces on the ground, who are supported by air strikes, fragmenting ISIS’ advance.
However ISIS has denied that the coalition strikes have hampered its activities.
Josh Ernest, a spokesman for the White House, described the progress made by the Kurds as "a sign of the critical importance the United States and its a partners’ ability, willingness and effectiveness to fight ISIS."
Ernst added that for this reason, Washington will continue to devote "significant resources" to assist factions fighting against ISIS.
Kurdish forces have also taken control of the Syrian city of Tal Abyad on the border with Turkey, which was previously held by ISIS. This development has been described as a strong blow to the militant organisation.
Tal Abyad was a major artery through which foreign fighters travelled to Syrian territory controlled by ISIS.
An official of the Syrian opposition National Coalition, based in Turkey, told the BBC on Monday that the YPG have 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 was to examine the situation of fleeing Syrian locals and listen to their testimony on the events in Tal Abyad.