Beirut (AFP) - The jihadist Islamic State group launched multiple attacks on government-held territory across northern and northeastern Syria on Thursday, a monitoring group said.

The Syrian Observatory for Human Rights said it was the first time that IS, accused by rival rebel groups of focusing its formidable firepower more on them than on the Damascus regime, had launched such coordinated assaults on army positions.

The jihadists went on the offensive near the main northern city of Aleppo, in Hasakeh province in the northeast and in Raqa province around their Euphrates valley stronghold, the Observatory said.

IS has been buoyed by sweeping victories north and west of Baghdad last month that prompted it to declare a pan-Islamic state straddling Iraq and Syria.

In Hasakeh province, the jihadists killed 11 soldiers in an attack on an army base, the Observatory said.

They also launched a brazen assault on the ruling Baath party provincial headquarters in Hasakeh city, over which witnesses said they saw the IS flag flying.

The mainly Kurdish city has remained under government control, although much of the surrounding countryside is under the control of Kurdish militia who have also been battling the jihadists.

In Raqa province, which is mainly held by IS, the jihadists attacked two positions of the army's 17th Division, sparking fighting that left dozens of dead and wounded, the Observatory said.

In one of the attacks, two suicide bombers, one a Saudi, blew up trucks packed with explosives.

IS posted photographs on its Twitter account of six decapitated corpses that it said were soldiers of the 17th Division.

East of Aleppo, the jihadists launched an assault on the Kuwayris airbase, sparking fierce fighting around the perimeter, the Observatory said.

The jihadists are seeking to expand the fiefdom they have carved out along the Euphrates Valley by taking the battle to neighbouring government-held areas after routing rival rebels in Deir Ezzor province in the east earlier this year, it added.