Kurdish-led forces in Syria seized a town from Islamic State on Tuesday after capturing a military base overnight, aided by U.S.-led air strikes in some of the most dramatic gains yet against the militants.
The Kurdish march deep into the heart of Islamic State territory follows their capture of a town on the Turkish border last week, halting the momentum of militants who had seized major towns in both Syria and Iraq last month.
The Kurdish YPG-led forces had taken full control of Ain Issa, said Redur Xelil, a spokesman for the group. The Syrian Observatory for Human Rights, which reports on the war, said Islamic State fighters had completely withdrawn from the town.
Overnight, the advancing forces took control of the nearby Liwa-93 military base, a strategic objective which Islamic State had captured last year from government forces.
The advance to Ain Issa brings the Kurdish forces and smaller Syrian rebel groups fighting alongside them to just 50 km (30 miles) from Raqqa, Islamic State's de facto capital, from which it rules over a "caliphate" across much of Syria and Iraq. Xelil said an assault on the city was not currently on the agenda.
The United States and its Arab and Western allies launched an air campaign against Islamic State last year after the fighters, also known as ISIS, ISIL or Daesh, proclaimed their "caliphate" to rule over all Muslims.
The YPG fighters have emerged as the most credible ally of the U.S.-led campaign on the ground in Syria, where Washington has fewer friends than in Iraq. Xelil described the U.S.-led air support as "excellent".
In a separate battle, the Syrian military and militias fighting alongside it have gained ground to the northwest of the city of Palmyra, which Islamic State captured from government control last month, according to the Observatory and a source in Syria briefed on the situation.
The West and its Arab allies have shunned the idea of partnering President Bashar al-Assad in the fight against Islamic State.
The YPG advance gathered pace last week when the Kurds, in partnership with smaller rebel groups under the banner of the Western-backed Free Syrian Army, captured Tel Abyad at the border, cutting an Islamic State supply route from Turkey.
Coordination between the YPG and the U.S.-led alliance has deepened since they first joined forces to defend the town of Kobani at the Turkish border, defeating Islamic State fighters in January after four months of battle there.
The Observatory said the aim of the YPG-led attack was to seize control of a highway running through Ain Issa which links the northeastern province of Hasaka with Aleppo.
Asked whether the de facto Islamic State capital was now a target, Xelil said: "Raqqa is a Syrian city, like Tel Abyad and Kobani, and all Syrians want it freed of Daesh terror. But at the current time it is not included in our agenda."
Daesh is an Arabic name for Islamic State.
The group has so far put up little resistance to the latest YPG-led advance. A Pentagon spokesman said last week Islamic State had appeared to "crack" at Tel Abyad.
Brett McGurk, the deputy U.S. special presidential envoy to the anti-Islamic State alliance, said on Twitter that YPG and FSA fighters were "making important gains" south of Tel Abyad, aided by coalition air support.
The Observatory said government forces had also advanced against Islamic State to the west of Palmyra, home to Roman ruins that are listed as a UNESCO World Heritage site.
Islamic State fighters consider such monuments to be idols that must be destroyed. They have planted explosives in the ruins, the Observatory reported on Saturday, raising concern the site could meet the same fate as monuments plundered and razed by the group in Iraq.
"We cannot yet say that this is the battle of Palmyra. It is a battle to fortify the regime's defensive lines," said the source in Syria briefed on the situation.