Hundreds of Sunni Muslim families fled Syria's coastal town of Banias on Saturday, fearing further sectarian violence after fighters loyal to President Bashar al-Assad killed dozens of people overnight, according to activists.

The activists said the killings in the Ras al-Nabaa district of Banias took place two days after state forces and pro-Assad militias killed at least 50 Sunnis in the nearby village of Baida.

The Syrian Observatory for Human Rights, a pro-opposition monitoring group, posted online a video showing the bodies of 10 people it said were killed in Ras al-Nabaa - half of them children.

Some were lying in pools of blood, and one toddler was covered in burns, her clothes singed and her legs charred.

Activist reports and videos on the killings could not be independently verified as the Syrian government restricts access for independent media.

The two-year-old uprising against four decades of Assad family rule has been led by Syria's Sunni Muslim majority, and sectarian clashes and alleged massacres have become increasingly common in a conflict that has killed more than 70,000 people.

Minorities such as the Alawites, an offshoot of Shi'ite Islam, have largely stood behind Assad, who is from the Alawite sect.

Banias is a Sunni pocket in the midst of a large Alawite enclave on Syria's Mediterranean coast, and activists in the area accuse militias loyal to Assad of ethnic cleansing.

"I estimate that hundreds of families left and headed for nearby towns like Jableh and Tartous," said Rami Abdelrahman, head of the British-based Syrian Observatory.

"But now the army is turning people back at the checkpoints outside the town, telling them to go back to Banias, that nothing is wrong. There are also announcements going out on mosque loud speakers telling people to return home."

Another video posted online by activists showed what they said were the bodies of 20 people killed in Banias overnight, all from the same family, including women and nine children.

The Observatory blamed the killings on the National Defence Forces (NDF), a new paramilitary group made up mostly of fighters from minorities that back Assad.

Trained and often directed by the military, the NDF describes itself as a reserve force for the army. The group has taken over the regionalised role of more informal Alawite militias known as shabbiha, which were accused of previous massacres of Sunnis.

On Thursday the Observatory reported the alleged killings in the village of Baida, just outside Banias. It said it had documented the names of 50 people killed but that it believed the final death toll could be between 100 and 200.

The group, which has a network of activists across Syria, declined to give a death toll for the killings in Banias.

Banias and Baida were the sites of some of the first sectarian clashes, when shabbiha fighters attacked Sunni street protesters in the first few months of the uprising, killing several people.

The hardline Sunni Islamist group Ahrar al-Sham published a video on Saturday of its fighters launching rockets they said were aimed at the village of Qurdaha, the birthplace of Assad's father, Hafez al-Assad, who ruled Syria for more than 30 years.

They said the attack was a response for the killings in Baida and Banias.