BEIRUT — Powerful Syrian Islamic rebel brigades announced Friday their merger into a single organization, a step meant to hold off surging government forces and stop rival groups from seizing more opposition-held territory.

The "Islamic Front" unites rebel groups who want to transform Syria into an Islamic state after they overthrow the government of President Bashar Assad, the groups said in a joint statement posted on the Tawhid Brigade's Facebook page.

The brigade is one of the most powerful groups to join the Islamic Front. The statement resembled a short video announcement by the group previously aired by satellite news network Al-Jazeera.

The merger of the Islamic groups also is meant to stave off challenges from the al-Qaida linked Islamic State in Iraq and the Levant, a powerful rebel brigade composed mostly of foreign Sunni fighters, said a spokesman and another activist close to the new group.

The spokesman declined to give his name because he was discussing internal affairs not meant to be shared publicly. The activist identified himself by the nom de guerre of Abdullah Hassan. Activists frequently give such names to protect themselves and their families from retribution.

Another powerful al-Qaida linked group, the Nusra Front, did not join the unified brigades. The spokesman said the Nusra Front wanted groups to join under its banner.

The spokesman said unifying the groups and their supply lines would take months because of communication problems challenging all of Syria's rebel forces.

Charles Lister, an analyst at IHS Jane's Terrorism and Insurgency Center, called the merger "an extremely significant development."

"The most militarily powerful Islamist rebel groups have effectively united their forces," Lister wrote in an analysis.

Lister estimated the merged group has a fighting force of at least 45,000 fighters. The groups include the powerful Ahrar al-Sham, the Tawhid Brigades and Suqour al-Sham.

The spokesman also said the Islamic Front wouldn't have relations with the Western-backed Syrian National Coalition. That coalition has seen its influence erode as rebels move away from the Turkish-based group toward generous Gulf donors.

Meanwhile, fighting raged Friday in the mountainous Qalamoun area north of Damascus that stretches to the Lebanese border, activists reported. The area is crucial for rebels to maintain smuggling routes to opposition-held areas south of Damascus and to the central city of Homs.

Since fighting there began last week, Assad's forces seized the town of Qara. On Friday, rebels seized the nearby town of Deir Attiyeh, severing the key Damascus-Homs highway that runs through the town, said Rami Abdurrahman of the British-based Syrian Observatory for Human Rights. An activist identified as Amer with the Qalamoun Media Center also confirmed the capture.

Al-Qaida rebels dominated the fighting, using waves of suicide car bombers, Abdurrahman said.

Some 45 miles away, fighting came close to a 40-foot (12-meter) statue of Jesus which stands close to an ancient monastery overlooking nearby rebel-held towns. Amer said Assad's forces positioned tanks and ammunition close to the statue to provoke sectarian tensions between mostly-Sunni rebels and other Syrian minority groups, including Christians.

Syria's state SANA news agency said mortars fired in the area wounded two people and damaged the Cherubim monastery. However, state media did not mention the massed weapons.

Also Friday, activists said a fierce battle near the northeastern city of Raqqa has killed some two dozen rebels so far as government troops still control a complex there home to the government's 17th Brigade.

Syria's conflict, now into its third year, has killed some 120,000 people, activists say. The crisis started as a peaceful uprising against Assad but deteriorated into all-out civil war after a government forces violently cracked down on protesters.