The extremist Islamic State group fought fierce battles yesterday with Syria's armed forces in a bid to seize control of Hasakeh, a key provincial capital in the country's northeast.

"Fierce clashes continued yesterday between regime forces and ISIS south of Hasakeh city. The regime is violently and intensely bombarding jihadist positions from the air," the Syrian Observatory for Human Rights said.

The Britain-based monitor said the regime was using barrel bombs - large containers packed with explosives -against jihadists edging towards the city, which is divided between Kurdish and government control.

Since their offensive began on May 30, ISIS fighters have advanced to the southern outskirts of Hasakeh using deadly suicide attacks and heavy mortar fire.

Citing a military source, Syria's state news agency SANA said the army had used "aerial weapons... to destroy equipment belonging to the ISIS terrorists".

The seven-day assault has killed at least 71 government loyalists and 59 extremists, including 11 who drove car bombs - IS's signature weapon - towards regime positions, the Observatory said.

The jihadists, which have expanded their control in central and eastern Syria and in neighbouring Iraq, seized a number of key posts, including a prison and power plant.

Hasakeh has since been without power, local activist Arin Shekhmos told AFP.