Fighters from the Islamic State in Iraq and the Levant (Isis) say they have overrun a large Syrian military base on the outskirts of the city of Raqqa.
The Islamist fighters have released images of captured soldiers being beheaded after the battle for the base.
The Syrian army did not confirm that the base had fallen, but said it was organising a counter-attack.
Isis already controls much of Raqqa province, and recently seized a swathe of territory in neighbouring Iraq.
The group, which has changed its name to Islamic State, describes the territory under its control in Iraq and Syria as a caliphate.
The Raqqa base, manned by Division 17 of the Syrian army, is said to have been captured overnight after coming under siege from Isis fighters.
According to the UK-based Syrian Observatory for Human Rights, a group that monitors casualties in the conflict, the assault on the base began with two suicide car bomb attacks. Meanwhile, Syrian army helicopters attacked Isis positions around the base.
Scores of Isis fighters and government soldiers were killed or hurt in the attack, the group said. The base is the largest of its kind in north-eastern Syria, and is said to be well-stocked with weapons and ammunition.
Rivals to Isis within the Syrian opposition had pointed to the lack - until now - of major confrontation over the base, close to the Isis stronghold of Raqqa, as a sign of collaboration between government forces and the militants.
However, BBC Arabic correspondent Rami Ruhayem says this narrative has been falling apart in recent weeks as clashes between Isis and the Syrian army have intensified.
The militants recently captured a gas field in the central province of Homs, in an attack that killed more than 200 people.
At least 170,000 people have been killed, a third of them civilians, since the start of the uprising against President Bashar al-Assad.
Some nine million people, or a third of Syria's pre-war population, have fled their homes.