By Erika Solomon

BEIRUT (Reuters) - The local commander of a Syrian rebel group affiliated to al Qaeda was freed on Sunday after being held by Kurdish forces in a power struggle between rival organizations fighting President Bashar al-Assad, activists said.

However, the pro-opposition activists gave conflicting reports of how the Islamist brigade commander in the Syrian town of Tel Abyad near the Turkish border had come to be free.

The British-based Syrian Observatory for Human Rights said Islamist rebels had exchanged 300 Kurdish residents they had kidnapped for the local head of their group, the Islamic State of Iraq and Sham (ISIS). Other activist groups challenged this account, saying Islamist fighters had freed Abu Musaab by force, with no Kurdish hostages released.

Sporadic fighting over the past five days in towns near the frontier with Turkey has pitted Islamists trying to cement their control of rebel zones against Kurds trying to assert their autonomy in mostly Kurdish areas.

The trouble highlights how the two-year insurgency against 43 years of Assad family rule is spinning off into strife within his opponents' ranks, running the risk of creating regionalized conflicts that could also destabilize neighboring countries.

The factional fighting could also help Assad's forces, who have launched an offensive to retake territory.

BELT OF TERRITORY

Assad has been trying to secure a belt of territory from Damascus through Homs and up to his heartland on the Mediterranean coast and, with the help of the Lebanese guerrilla group Hezbollah, has won a string of victories in Homs province and near the capital.

On Sunday his forces ambushed and killed 49 rebels in the Damascus suburb of Adra, the Observatory said.

The town was once a critical point along the route used by rebels to bring weapons to the capital, but Assad's forces recaptured it a few months ago and have been working to cut off rebel territories in the area.

To the north, activists reported Turkish troops reinforcing their side of the frontier near Tel Abyad, but the army could not be reached for comment. Turkish forces exchanged fire with Syrian Kurdish fighters in another border region earlier in the week.

The Observatory said the alleged prisoner exchange was part of a ceasefire agreed after a day of fierce clashes in Tel Abyad, but other activists said there was no deal and reported that many Kurdish residents were being held by ISIS fighters.

The Observatory said the fighting in Tel Abyad started when the local ISIS brigade asked Kurdish Front forces, which have fought with the rebels against Assad, to pledge allegiance to Abu Musaab, which they refused to do.

Other activists said the clashes were an extension of fighting that broke out last week in other parts of the northern border zone.

Opposition activists also reported the killing of at least 13 members of a family in the Sunni Muslim village of Baida on Sunday, in what they described as a second sectarian massacre there.

FIGHTING NEAR THE COAST

The killings followed a rare eruption of fighting between Assad's forces and rebels in the coastal province of Tartous, an enclave of Assad's Alawite minority sect that has remained largely unscathed by the civil war.

Syria's marginalized Sunni majority has largely backed the insurrection while minorities such as the Alawite sect, an offshoot of Shi'ite Islam, have largely supported Assad, himself an Alawite.

The Observatory said four women and six children were among those killed in Baida.

"A relative came to look for them today and found the men shot outside. The women's and children's bodies were inside a room of the house and residents in the area said some of the bodies were burned," said Rami Abdelrahman, head of the Observatory.

In May, pro-Assad militias killed more than 50 residents of Baida and over 60 in the nearby town of Banias. In those killings, some bodies, many of them children, were found burned and mutilated.

The anti-Assad revolt has evolved from its origins as a peaceful protest movement in March 2011 into a civil war that has killed over 100,000 people and turned markedly sectarian.

The ethnic Kurdish minority has been alternately battling both Assad's forces and the Islamist-dominated rebels. Kurds argue they support the revolt but rebels accuse them of making deals with the government in order to ensure their security and autonomy during the conflict.

The Kurdish people, scattered over the territories of Iran, Turkey, Iraq and Syria, are often described as the world's largest ethnic community without a state of their own.

(Additional reporting by Isabel Coles in Arbil and Jonathan Burch in Ankara; Editing by Kevin Liffey)