Kurdish fighters and their allies have captured a Syrian military base once held by the Islamic State group (ISIS), activists and officials said Tuesday, moving within some 30 miles of the extremists' de facto capital.

Taking the Brigade 93 base further squeezes the extremists, especially after they lost a major supply line when the Kurds captured the town of Tal Abyad on the Turkish border last week.

However, even with the aid of U.S.-led airstrikes, battling even closer to the Islamic State's stronghold of Raqqa could prove costly for the Kurds and allied rebel factions.

The Britain-based Syrian Observatory for Human Rights and Kurdish activist Mustafa Bali said Kurdish fighters from the People's Protection Units, or YPG, captured the base Monday night. Both said YPG fighters and their allies later entered parts of the nearby town of Ein Issa, the last major residential area north of Raqqa, which ISIS considers the capital of its self-declared "caliphate" across Syria and Iraq.

The YPG's official Facebook page said "dozens of Daesh mercenaries were killed" at Brigade 93, using an Arabic acronym for the extremist group. The Observatory said that ISIS militants transferred the corpses of 26 of its fighters to Raqqa after they were killed in Ein Issa by airstrikes.

The U.S. has found a reliable partner in the YPG, who have been the main force in the battle against ISIS in Syria. They 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."

ISIS, while losing ground in Syria, largely has held onto its territory in Iraq. They also continue to have a supply line to Turkey running through northwestern Syria to Raqqa. It's not clear whether the Kurds will push in further on Raqqa. When cornered in the past, the militants have relied on coordinated mass suicide car bomb attacks and other scorched-earth tactics.