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.

The head of Syria's former Al-Qaeda affiliate in a video released Sunday urged supporters to "take up weapons" to defend his group's bastion of Idlib against increased bombardment.

UN-linked aid groups have suspended activities in parts of violence-plagued northwest Syria, where stepped up bombardment by the regime and Russia is jeopardising the safety of humanitarian workers.