The villages of Nubol and Zahraa in war-battered Arbil province have been under rebel siege for a year and a half.
On Saturday evening, the rebels and their allies from Al-Qaeda affiliate Al-Nusra Front launched a major attack aimed at taking over the villages, though their bid has so far failed, according to the Syrian Observatory for Human Rights.
"Fierce clashes have raged since midnight on the edges of Nubol and Zahraa," it said.
The Britain-based group described the current offensive as "the most violent" since the rebels and Al-Qaeda laid siege to the Shiite villages.
Observatory director Rami Abdel Rahman also told AFP it was the first time that Al-Nusra Front had made advances in the area.
The jihadists announced their role via Twitter.
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"The mujahedeen (have launched) an offensive against the rawafed in the villages of Nubol and Zahraa," said Al-Nusra Front via Twitter, using a pejorative term to refer to Shiites.
At least eight rebels and jihadists, along with one civilian, have been killed since Saturday night, the Observatory said.
Syria's war began as a peaceful revolt, but later morphed into a Sunni-dominated insurgency after President Bashar al-Assad's regime launched a massive crackdown on dissent.
The majority of Syria's rebels, like its population, are Sunni, while Assad belongs to the Alawite community, an offshoot of the Shiite sect.
The main forces defending Nubol and Zahraa are pro-regime militiamen, including fighters from Lebanon's Shiite Hezbollah movement.
Activists say previous rebel bids to take over Nubol and Zahraa failed after the rebels were pressured by their US backers into retreating, for fear of sectarian massacres.
Several aid convoys have entered Nubol and Zahraa in recent months.