Syrian Kurdish forces and allied rebel fighters have advanced to within 30 miles of the Islamic State's self-declared capital, capturing a key military base on Monday and a nearby town on Tuesday.
The Kurdish People's Protection Units (YPG), backed by an alliance of rebel factions and coalition airstrikes, seized the Brigade 93 base on Monday night after heavy clashes and went on to push IS out of the town of Ain Issa directly north of its stronghold of Raqqa, according to the UK-based Syrian Observatory for Human Rights (SOHR). The Observatory, which collects information from a network of local sources, added that the YPG and opposition groups were now clearing up "huge quantities" of Improvised Explosive Devices and mines planted by IS when it retreated.
The YPG said it had killed "dozens" of IS members at Brigade 93 on its official Facebook page and also said that it had begun to dismantle mines left by IS.
Observatory and YPG spokesman Redur Khalil told the Associated Press: "Ain Issa and dozens of villages around it are under our control." The next task was to enforce and protect these areas because the YPG know that the IS will strike back, he said.
It is another major blow to IS after the YPG took complete control of the town of Tal Abyad on the Syrian-Turkish border last week, cutting off a major supply line.
Tal Abyad is less than 60 miles from Raqqa, and directly across the border from the Turkish town of Akcakale, from where the Islamic State had brought in fighters, ammunition and supplies, through official and unofficial crossings. The jihadists' closest alternative supply route is likely the city of Jarablus in Aleppo province. Food prices in Raqqa have now soared due to losing touch with the Turkish border, SOHR said.
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Ain Issa is the last major residential area before Raqqa, but further progress towards the city that IS consider the capital of its "caliphate" will likely not be so swift.
"Operations will continue, but it is imperative that we first attempt to secure areas under our control," said Nawaf Khalil, head of the Germany-based Kurdish Center for Studies. "Raqqa is a vast area and attacking it will need a great deal of coordination with other groups and the international alliance."
The battle for Tal Abyad caused more than 16,000 people to attempt to flee into Turkey, reported the BBC, although many have now begun to return to their homes after Turkish authorities re-opened the border gate.
Meanwhile, Syrian state-linked media reported on Monday that the government troops had retaken some ground from IS near Palmyra. SOHR said regime forces were now just six miles from the city, but the jihadists are reported to have mined its historic ruins.
The Associated Press contributed to this report.