Fighters from the extremist Islamic State group today overran part of an army base in northern Syria, which has been under the militants' siege for months, in ferocious battles that killed or wounded dozens on both sides, activists said.

The battle over the base is the latest in the Islamic State's push to capture as much of Syrian territory as it can. Since June, the group has seized a huge chunk of territory straddling the Iraq-Syria border, where they have declared a self-styled caliphate.

The base lies in Raqqa province, where much of the territory fell to the Syrian opposition last year. Earlier this year, the Islamic State, which sided with the rebels at the start of the Syrian conflict three years ago, captured much of Raqqa and has tried to capture the base several times.

The assault on the base began around midnight on Wednesday with two suicide car bombs, said the Britain-based Syrian Observatory for Human Rights. Syrian army helicopters fought back, targeting jihadi positions around the base, the activist group said.

The Observatory said 35 Islamic State fighters died and that dozens of government troops were killed or wounded, including six soldiers who were beheaded. It said both sides exchanged mortar and artillery fire.

The Syrian air force carried out 12 raids around Division 17 and the nearby provincial capital of Raqqa, which is also controlled by the Islamic State, said the Local Coordination Committees, another activist group. It said army helicopters were dropping barrel bombs around the base.

Elsewhere today, Islamic State fighters stormed the headquarters of Syrian President Bashar Assad's ruling Baath party in the northeastern city of Hassakeh in the predominantly Kurdish province that carries the same name, activists said.

Black banners of the Islamic State were seen raised over the Baath party building, the Observatory said.

Juan Mohammed, a Kurdish official in Hassakeh, said there were two explosions near the building but he added that he had no immediate information about whether the Islamic State had stormed the Baath headquarters.

In their push, the Islamic State fighters have also captured much of Syria's oil-rich eastern province of Deir el-Zour, which borders Iraq. Last week, the jihadis captured a gas field in the central province of Homs, an attack that left more than 200 people dead.

Clashes between government forces and Islamic State fighters have been rare until after the group's blitz advance in northern and western Iraq in June. Since then, violence between the two intensified as jihadis try to remove all rival groups from areas under their control.

Assad's forces have fought back, but for now appear mostly intent on consolidating the territory firmly under their control in his powerbase, the capital of Damascus, and in his Allawite heartland to the west.