Kurdish-led forces say they have captured a military base from the Islamic State in Syria's Raqqa Province only a week after seizing a key border crossing from the militant group.
The Kurds, aided by U.S.-led air strikes and smaller Syrian rebel groups, then pushed to within 4 miles of Ain Issa, a town 30 miles north of Raqqa city, Islamic State's main seat of power, said Kurdish spokesman Redur Xelil.
IS had held the military base, Liwa 93, since capturing it from the Syrian military last year.
"This means that the Islamic State keeps collapsing inside its own stronghold," said Rami Abdulrahman, head of the Syrian Observatory for Human Rights, which monitors the conflict.
The rapid advance into Raqqa Province has defied expectations, after a four-month battle for the border town of Kobani, which the Kurds finally won in January.
The Kurdish front in northern Syria has been one of the few areas of success for the global coalition against IS.
The Pentagon said that IS forces appeared to crack at the border town of Tel Abyad, which fell to Kurdish People's Protection Units in less than two days last week, cutting a key route for IS supplies and foreign fighters coming in from Turkey.