BEIRUT — Backed by US-led airstrikes and buoyed by battlefield successes, Kurdish fighters kept up an offensive through northern Syria on Tuesday, driving Islamic State militants out of a town near the extremists' de facto capital of Raqqa.
The capture of Ein Issa came just hours after the Kurdish forces had overrun a nearby military base, increasing the pressure on the Islamic State group less than two weeks after it lost the strategically located town of Tal Abyad on the Turkish border, severing a vital supply line.
The advances by the Kurdish fighters in Syria as well as in northern Iraq has been credited largely to a high level of coordination between the ground forces and the nearly year-old air campaign being led by Washington against the Islamic State, also known as ISIS or ISIL in English and by its Arabic acronym, Daesh.
White House Press Secretary Josh Earnest called the success by the Kurds "an indication of how critically important it is for the United States to have a capable, willing and effective partner fighting ISIL on the ground."
That was why the US was dedicating "significant resources" to building up opposition forces, he said. That work was "a more difficult task" in Syria than Iraq, but that "this is a pretty good illustration of why that very difficult work is important," Earnest added.
Ein Issa is only 50 kilometers (30 miles) north of Raqqa, the stronghold of the Islamic State's self-declared caliphate that spans parts of Syria and Iraq.
The recent battlefield setbacks for ISIS were mentioned in an audio message by the group's spokesman, Abu Muhammad al-Adnani, for the Muslim holy month of Ramadan.
"God never gave the mujahedeen a promise of victory every time," al-Adnani said in the message posted on social media, adding that the faithful "may lose a battle or battles and may lose towns and areas, but will never be defeated."
He urged Sunni Muslims to use the time of piety and dawn-to-dusk fasting as an occasion to wage jihad and seek martyrdom.
"Attack them everywhere and shake the ground beneath them," he said in the morale-boosting message, his voice rising. "If you lose territory, you will win it back and more in the future, God willing."
It was not immediately possible to verify the recording, but it resembled previous audio statements from the group.
The capture of Ein Issa does not necessarily mean that the immediate next move by the Kurdish forces and their allies will be to march on Raqqa, but it appears to have unsettled some of the city's ISIS-linked residents.
The wives and children of ISIS fighters were leaving Raqqa in droves Tuesday in anticipation of a possible attack, while the militants have redeployed to positions to better defend the city, according to the Britain-based Syrian Observatory for Human Rights. The group gets its information from a network of activists across Syria.
Its director, Rami Abdurrahman, told The Associated Press that a band of about 20 ISIS fighters who had hidden in Ein Issa when the Kurdish troops stormed it have re-emerged, and battles were raging in the city. Other groups of IS fighters were also engaging the Kurds on the outskirts. He had no casualty figures and there was no immediate confirmation of continuing fighting.
The first step toward capturing Ein Issa came Monday night when its "Brigade 93" — a Syrian army military base that had been captured by the ISIS — was taken over by the Kurdish forces, according to the observatory and Kurdish activist Mustafa Bali.
"Ein Issa and dozens of villages around it are under our control," said Redur Khalil, a spokesman for the Kurdish People's Protection Units, or YPG. The next task is to reinforce and protect these areas because they know ISIS will strike back, he told AP.
The Observatory said the YPG and its allies are also trying to gain control of a key highway linking Aleppo, Syria's largest city, with the northeastern city of Hassakeh.
The YPG's official Facebook page said "dozens of Daesh mercenaries were killed" at the Brigade 93 base. The Observatory said the bodies of 26 ISIS fighters killed by airstrikes in Ein Issa were taken to Raqqa.
"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 US has found a reliable partner in the YPG, which has been the main force battling the Islamic State group in Syria. The Kurds are moderate, mostly secular fighters, driven by revolutionary fervor and deep conviction in their cause. They are backed by Arab tribesmen, Assyrian Christian gunmen and members of the rebel faction known as Burkan al-Furat — Arabic for the "Volcano of the Euphrates."
"They are a cohesive, well-trained and highly motivated force," said Mutlu Civiroglu, a Washington-based expert on Kurdish affairs. "They are defending their homes and they know the terrain well, unlike the foreign fighters who have joined the ISIS."
By contrast, Iraqi government forces and allied Shiite militiamen have been slow in retaking ISIS-held territory. The Iraqis have also suffered occasional losses.
The Iraqis drove IS fighters from Saddam Hussein's hometown of Tikrit in April, but lost Ramadi, the capital of Anbar province west of Baghdad, last month. It was a repeat of the meltdown government forces suffered when ISIS militants swept across much of the nation's north and west last summer.
It was unclear whether the Syrian Kurds will try to push farther toward Raqqa. Despite the Kurds' recent gains, the Islamic State militants still have another supply line from Turkey that runs through northwestern Syria to Raqqa.
When cornered in the past, the militants have used coordinated mass suicide car bombings and other scorched-earth tactics. The Observatory reported at least four suicide attacks Tuesday against Kurdish and allied forces in northern Syria. Initial reports said at least three Kurdish and allied fighters were killed.
Syria's official SANA news agency said two suicide bombings struck a roundabout in the northeastern city of Hassakeh, killing one person and wounding 13, including five children.
Also Tuesday, a media arm of the Islamic State group in Iraq posted a video purporting to show the killing of over a dozen men it described as spies by drowning them in a cage, decapitating them with explosives, or firing a rocket-propelled grenade at them.
