BEIRUT — Backed by U.S.-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 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.

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 U.S. 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 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 the Islamic State 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.”

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 Islamic State-linked residents.