Air strikes by a US-led coalition against the Islamic State group in northeastern Syria have killed around 170 jihadists in the past 48 hours, a monitoring group says.

"The jihadists were killed in the past 48 hours in the province of Hasakeh, nearly all of them in very intense air strikes by the international coalition which is helping Kurdish forces in the area," said the Syrian Observatory for Human Rights on Tuesday.

In its latest update on the coalition's operations, the US military said in a statement that the alliance's warplanes carried out seven strikes near Hasakeh.

They struck four IS tactical units, destroying seven vehicles, three fighting positions, two armoured vehicles and a shipping container.

"In northern Syria, Daesh continues to cede military capacity, fighters and terrain," US Brigadier General Thomas Weidley said, using the Arabic acronym for IS.

The report came hours after rebels including Al-Qaeda's local affiliate seized the Syrian regime's largest remaining military base in northwestern Idlib province.

The loss of Al-Mastumah base leaves only a few positions in regime hands in the province, which lies along the border with Turkey and neighbours the government stronghold of Latakia province on the Mediterranean coast.

"All regime forces have withdrawn from Al-Mastumah, the largest regime base in Idlib, leaving it completely in the hands of opposition fighters," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP.