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 six large 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 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.

"God alone in his glory will be our reference," the Islamic Front said in its statement.

The merger of the Islamic groups also is meant to stave off challenges from the al Qaeda 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 Qaeda linked group, the Nusra Front, did not join the unified brigades. The spokesman said the Nusra Front wanted groups to join under their banner.

So far the merger is just in name, the spokesman said. He said unifying organization structures, fighters and weapons flows 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 Ahrar al-Sham, the Tawhid Brigades and Suqour al-Sham -- three powerful Syrian fighting groups.

The spokesman also said the Islamic Front would have no relations with the Western-backed Syrian National Coalition. That coalition has seen its presence within the country all but disappear as rebels move away from the Turkish-based group toward generous Gulf donors.

The Islamic Front announcement also comes after the charismatic leader of the Tawhid Brigade died Monday from wounds he suffered in a government airstrike last week. Abdul Qadir Saleh founded the Tawhid Brigade, which boasts an estimated 10,000 fighters and spearheaded fighting that seized large sections of the northern city of Aleppo last year.

Lister said the announcement of a unified group would serve to rally rebels despondent over Saleh's death and battlefield losses.

Meanwhile, fighting raged Friday in the mountainous Qalamoun area north of Damascus that stretches to the Lebanese border, activists reported. Since fighting there began last week, Assad's forces have seized the town of Qara, but have faced rebels conducting suicide car bombings. The Qalamoun area is crucial for rebels to maintain smuggling routes to opposition-held areas south of Damascus and to the central city of Homs.

Also, activists said Friday a fierce battle near the northeastern city of Raqqa has killed 24 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 brutal government crackdown on dissent. In the past year, the fighting has taken on sectarian overtones.