Syrian rebels fired around 1,000 rockets, mortar shells and homemade projectiles at two besieged Shiite towns in Idlib province, a monitor said Monday.
The Syrian Observatory for Human Rights said there were reports of dead and injured in the heavy fire on Fuaa and Kafraya that started on Sunday night, but it had no confirmed toll.
Fuaa and Kafraya are among the last regime-held outposts in the northwestern province of Idlib, most of which has been captured by an Islamist rebel alliance that includes Al Qaeda affiliate Al-Nusra Front.
The alliance, known as the Army of Conquest, began an attack against the villages on July 15, saying it was retaliation for a regime offensive on Zabadani, the last rebel-held bastion along Syria's border with Lebanon.
It said the attack would "give you a taste in the north of what our people are tasting in Zabadani."
Al-Nusra and many of its hardline allies consider Shiite Muslims to be heretical.
Elsewhere, the Observatory and Syrian state media reported deaths in rebel fire on the government-controlled portion of Aleppo city in northern Syria.
The Observatory said four people were killed, among them two children, and 20 others were wounded in the rebel shelling.
State media put the toll at three dead in the city, which has been divided between government control in the west and rebel control in the east since shortly after fighting erupted in Aleppo in mid-2012.
ISIS-free zone imminent?
Meanwhile, Al-Nusra Front has announced a surprise withdrawal from its front lines against its jihadist rival Islamic State (ISIS or IS) in areas along Syria's northern border with Turkey.
It said the decision was being taken to avoid cooperating with a US-Turkish plan to create an ISIS-free zone in northern Aleppo province, along the Turkish border.
"We announce our withdrawal from our front lines with the 'khawarij' [Arabic for apostates, a term both ISIS and Nusra both use against each other – ed] in northern parts of Aleppo province," the group said in an online statement published Sunday.
It did not specify which parts of the frontline with ISIS the group would withdraw from.
Al-Nusra is a key opponent of ISIS in the northern province of Aleppo along with its powerful Islamist ally Ahrar al-Sham.
The group said it was withdrawing in response to a US-Turkish decision late last month to implement an ISIS-free zone.
"Al-Nusra Front does not see entering this alliance, helping it, or coordinating with it as legitimate," the statement read.
It said the decision to create the ISIS-free zone was "not a strategic decision… made by the fighting groups, rather its primary goal is Turkey's national security."
Turkey, a key backer of a range of rebel groups in Syria, has agreed with Washington to work on expelling ISIS from a swathe of territory in northern Aleppo province.
The US-led coalition fighting ISIS, which now has access to Turkish bases for the first time, will provide air support, but the nature of the ground forces fighting against IS remains unclear.
But Turkey has previously been accused of turning a blind eye to ISIS activity on its own soil and along its border, and evidence suggests Ankara's Islamist government has been directly backing other jihadist groups, including Al-Nusra Front.
The Syrian Observatory for Human Rights monitoring group said Al-Nusra had so far fully withdrawn from the villages of Dalha and Harjaleh, which lie near the Turkish border.
"There are other villages that they officially withdrew from, but they stayed in the hands of Nusra's allies," said Observatory head Rami Abdel Rahman.
Aaron Stein, a non-resident fellow at the Atlantic Council research institute, said Al-Nusra's withdrawal could make it easier for Washington to work with ground forces in the would-be ISIS-free zone.
"The way I'm looking at it is Turkey influenced Ahrar al-Sham, which put pressure on Al-Nusra… to remove itself from the areas proposed in the buffer zone," Stein told AFP.
The Al-Nusra withdrawal makes it "more palatable for the US to give more support to rebel groups there," he added.
Few details have emerged on which armed opposition forces Ankara and Washington will partner with to enforce their ISIS-free zone.
Even if ground forces don't receive direct military support from the US, Stein said they could benefit from US-led air cover over the border area.
Despite the withdrawal, Al-Nusra will continue to have "influence" and allies – particularly Ahrar al-Sham – in the zone in question, Stein said.
Syria's conflict began with peaceful demonstrations in March 2011, but spiraled into a multi-front war that has become increasingly dominated by jihadist forces.