Syrian government air strikes on the rebel-held northwest killed four civilians, two of them children, on Saturday, a war monitor said, as a two-month flare-up showed no let-up.

The Idlib region of some three million people is supposed to be protected by a September buffer zone deal, but the jihadist-run enclave has come under mounting bombardment by the government and its ally Russia since late April.

Fighting raged in northwest Syria on Thursday as clashes between regime forces and jihadist-led fighters killed more than 100 combatants in two days, a war monitor said.

The Idlib region, home to some three million people, is supposed to be protected by a months-old international truce deal, but it has come under increased bombardment by the regime and its Russian ally since late April.

A Syrian refugee in the US was arrested Wednesday on suspicion of planning an attack against a Pennsylvania church in the name of the Islamic State group, the Justice Department said.

Syrian air defence shot down Israeli missiles targeting the south of the country Wednesday, state media said, as a monitor reported positions of the regime's Lebanese ally Hezbollah had been hit.

The United States and Russia are in talks on a "potential way forward" that would see Syria come out of international isolation if it agrees to a series of steps including a ceasefire in Idlib, the U.S. envoy for Syria said.

Outside her makeshift tent, Syrian mother Mona Mutayr lays out a frugal meal of potatoes and cucumber as her family marks their first Ramadan since being displaced by war.

Syrian government air strikes killed 15 civilians, 12 of them in a market, as fierce fighting raged for the jihadist-held northwest, a monitor said on Wednesday.

Dozens of international aid groups say conditions in Syria's northwest have reached "crisis point" and are calling for an immediate end to the fighting.

The aid groups said, in an appeal issued Friday, violence has displaced an estimated 180,000 people in the last two weeks.

Syria called Friday on all parties to exercise "restraint" in the Gulf as its ally Iran comes under mounting pressure from the United States, state media said. 

Turkish President Recep Tayyip Erdogan accused the Syrian regime of "seeking to sabotage" Ankara's relationship with Russia through its latest offensive in the northwest of the war-torn country.