Washington designates group as a terrorist organisation, saying it is trying to hijack rebellion on behalf of al-Qaeda.
Scores of civilians from the minority Alawite community of which President Bashar al-Assad is a member have reportedly been killed in the central Syrian province of Hama.
Bomb attacks in the village of Aqrab killed or wounded at least 125 civilians, said the UK-based Syrian Observatory for Human Rights (SOHR), which could not immediately give a breakdown of the casualties.
“We cannot know whether the rebels were behind this attack, but if they were, this would be the largest-scale revenge attack against Alawites,” said observatory director Rami Abdel Rahman.
Al Jazeera’s Rula Amin, reporting from Beirut, said: “What happened there is obviously for many people in Syria an alarming sign, especially because many of those who have been killed are Alawites.”
Aqrab is located near Houla, a majority Sunni Muslim village where 108 people, including 49 children and 34 women, were massacred on May 25 in what was widely blamed on pro-regime armed men despite denials from Damascus.
Earlier on Tuesday, Syrian rebels took full control of a military base near the city of Aleppo that they stormed two days ago, killing at least 35 government troops in the fighting, the SOHR said.
The group said the battle for the Sheik Suleiman base ended after rebels took over the main compound and warehouses at the site.
The rebels first entered the base on Sunday afternoon, after weeks of fighting with soldiers loyal to Assad.
The SOHR said 64 government troops were also injured in the assault.
Among the groups that seized the base was the al-Nusra Front, which the US government on Tuesday blacklisted as a “terrorist organisation”. The US says that al-Nusra is allied to al-Qaeda in Iraq.
Military school attacked
Also on Tuesday, rebel troops attempted to wrest control of a military infantry academy near the northern entrance of Aleppo from government forces.
The military school, located close to the town of Muslimiyeh near the Hanadarat Palestinian refugee camp, houses approximately 3,000 soldiers, the SOHR said.
Fierce clashes have raged since rebels encircled the academy just over two weeks ago.
The Observatory also said that 11 people were killed elsewhere in the province, including three children and two women, on Monday night when four mortars struck the Sheikh Maksoud district of the city of Aleppo.
Another 13 people were wounded, some critically, in the bombardment on the largely Kurdish neighbourhood.
The mortars were fired by rebels from the Bustan al-Basha district, according to activists.
In Damascus, the army shelled southern districts of the city while security forces raided several areas of the nearby Midan district, the Observatory said.
Further south of the capital, two men were killed in bombing on the town of Daraya, where rebels have been resisting a military onslaught since mid-summer, while shelling was also reported across the eastern outskirts of the city.
The Local Co-ordination Committees, a network of activists on the ground, meanwhile, reported that fierce government shelling was targeting southern areas of Damascus.
Fighting was also reported on Tuesday in the cities of Jisr al-Shughur, Arbeen and Deir al-Zor.
Refugee numbers soar
The latest UNHCR figures, released on Tuesday, showed that more than 500,000 Syrians have fled their country for other areas in the region.
“According to UNHCR’s latest figures for Lebanon, Jordan, Iraq, Turkey and North Africa, 509,559 Syrians are either already registered (425,160) or in the process of being registered,” the UNHCR said in a statement issued in Geneva.
The number of registered Syrian refugees region-wide rose by about 3,200 per day in November, and close to 1,000 Syrians crossed into Jordan during the past two nights alone, it said.
Lebanon is now host to 154,387 registered Syrian refugees who have fled the 20-month-old conflict, Jordan has 142,664, Turkey 136,319, Iraq 65,449 and North Africa 11,740, the statement said.
In addition, large numbers of Syrians have crossed into neighbouring countries but have not yet come forward to register for refugee status and assistance, it said.