Islamic State militants launched assaults on Syrian forces across three provinces on Thursday that killed key government figures, including two brigadier generals, said activists and residents, in a rare confrontation between the two sides during the war.

In one assault, the jihadists besieged two military bases outside Hasakah city in Syria's east, killing a commanding brigadier general, according to the Syrian Observatory for Human Rights, an activist group in London with a network of contacts across Syria.

The militants, wearing military uniforms, also stormed the headquarters of the ruling Baath Party, where they killed Hanna Atalla, a party leader in the city, according to the Observatory and residents in interviews. The Islamic State militants flew their black flag over the building, which had served as a government military operations center.

Also in Hasakah, the militants killed Abdul Samad al-Nazzal, a senior officer with the National Defense Forces, a pro-regime paramilitary group, these people said.

The Syrian government acknowledged that its Baath Party headquarters were attacked but didn't comment on any battles or deaths.