Syrian rebels today were defending Marea, one of their main strongholds in the north, against a rapid advance by Islamic State jihadists, a monitoring group and an activist said.

The opposition Syrian National Council in exile called for the international community to attack IS positions not only in northern Iraq but also in Syria.

IS fighters were nearing the towns of Marea and Aazaz, both bastions of rebel groups battling Syrian President Bashar al-Assad's regime, after seizing a number of villages in Aleppo province bordering Turkey in just three days.

If the jihadists manage to take the strategic towns, this could deal a severe blow to rebel groups battling both regime forces and the IS.

"There is fierce fighting on Saturday between rebels and the IS around Marea," the Syrian Observatory for Human Rights said in a statement.

Marea is the main base of the coalition Islamic Front, the most important rebel grouping that is fighting both Assad's troops and the jihadists.

"The pressure is now on Marea, militarily," said activist Abu Omar, spokesman for the Marea "revolutionary council".

"The rebels have sent in many reinforcements and weapons to the area in and around Marea," he told AFP, speaking via the Internet.

"The rebels consider this to be one of the most important battles against the IS... There's no question of losing," he added.

Abu Omar said the jihadist group, which is also currently being targeted by US air strikes in northern Iraq, is using heavy weaponry it seized when it routed the Iraqi army during a lightning offensive there in June.

The arms were supplied to the Iraqi armed forces by the United States, experts say.

"They're using their tanks and artillery to attack towns and villages" in Syria, Abu Omar added.

Syria's opposition in exile late Friday warned that the danger posed by IS jihadists in Iraq and Syria was "indivisible".

"If the international community is serious in fighting against it (IS), such fighting must not be limited to Syria," it said in a statement.

The clashes near Marea comes after IS fighters took around 10 villages in northern Aleppo on Wednesday and Thursday, the Britain-based Observatory said.

It said the fate of dozens of rebels captured during the jihadist offensive remained unknown after nine were beheaded on Wednesday and another eight today in Akhtarin.