Syrian rebel forces killed a pro-regime Sunni cleric in the city of Aleppo, with some reports suggesting he was beheaded, and then dragged his body through the streets, a watchdog group said on Saturday.

Sheikh Hassan Seifeddin, imam of a mosque in the northern Aleppo neighborhood of Sheikh Maqsud, "was killed overnight Friday by rebel fighters in the east of the area and his body was dragged through the streets," the Syrian Observatory for Human Rights said.

Syrian state news agency SANA reported that Seifeddin was "assassinated by terrorists who mutilated his body afterwards," while official television station al-Ikhbariya said he had been "slaughtered" and beheaded.

"The ulema (clerics) of Aleppo denounce this despicable crime committed by the enemies of humanity who assassinated Sheikh Hassan Seifeddin and laid his head on the minaret of al-Hassan mosque in Sheikh Maqsud," the station reported.

Sheikh Maqsud is a majority Kurdish district of Aleppo, and fierce battles between rebels and regime forces have been raging since Friday in its eastern quarter where much of the district's non-Kurdish Sunni residents live.

The ulema called on the Syrian army to "liberate Syria from the criminal mercenaries with obscurantist ideas," in an apparent reference to hardline jihadist groups amongst the armed opposition.

A March 22 suicide bomb attack on a central Damascus mosque claimed by jihadists killed 42 people, including the country's most prominent pro-regime Sunni cleric.

The Observatory said 31 people had been killed in Sheikh Maqsud in 24 hours, including 10 civilians, 14 pro-regime gunman and seven rebels.

At least 157 people were killed through Syria on Friday, according to the watchdog group which gathers its information from a network of activists and medics on the ground.