Two of these groups, Division 13 and Fursan al-Haq, have gained grounds while fighting alongside a group calling itself Army of Fatah, an Islamist alliance that includes the Nusra Front – al-Qaeda's wing in Syria – and another hardline militant group, Ahrar al-Sham.
Division 13 and Fursan al-Haq, which do not share a joint command structure with Army of Fatah and reject its anti-Western views, receive covert support from the CIA, senior group members said in interviews with Newsweek.
In Syria, US-backed moderate rebel groups leading the fight against President Assad are falling, with some of them joining radical Islamist factions.
Hazzm, a rebel brigade that received anti-tank weapons from the United States, was crushed in March by Nusra, which also seized the group’s American weapons. That has left Washington for months without a reliable rebel force to publicly support.
"Maybe Nusra fought… Hazzm at the start, because they said they were no good. Now they have a plan to fight only the [Assad] regime," Mazin Qusum, commander of the Siham Al-Haqq brigade, an Islamist unit fighting alongside the Army of Fatah, told Reuters in the Turkish border town of Reyhanli.
One of Ahrar al-Sham’s founders, Abu Khaled al-Soury, fought alongside Osama bin Laden and was close to al Qaeda's current chief Ayman al-Zawahri. While such close ties preclude public backing from Washington, these groups are stressing unity.
In a sign of cooperation, an arm of the Western-backed opposition government in exile, the Syrian National Coalition, has been given the responsibility for health and education in areas the rebels have recently captured, Newsweek reported.
Abu Hamoud, a commander from Division 13, said his group is coordinated with Nusra Front, which the United States considers a terrorist organization, but this does not mean it is aligned to it.
"It is completely stable in Idlib province (where Army of Fatah has seized towns), there will be no fighting between the brigades," Abu Hamoud was quoted as saying by Newsweek.
It remains unclear, however, if there will be conflict within the Islamic alliance after liberating some areas, and if those groups would impose Sharia law there.
"We and the others, when we liberate all of Syria, we will meet and determine what kind of law there will be," Abu Mohammed, a representative of Ahrar al-Sham's political office, said as he sat with Division 13 and Fursan al Haq commanders in Istanbul, Turkey.
Some fighters warn that groups could turn against each other once they reach their military objectives, with one member from the Islamist group Liwa Tawheed saying “it's very possible there will be power conflicts among them.”
"As long as there are strategies to attack the regime and major strategies to work together, there won’t be a problem because the brigades will have a common enemy," that same fighter told Newsweek.