ALEPPO, Syria, Nov. 17 (UPI) -- An array of rebel groups in northwestern Syria's Aleppo and Idlib provinces announced Tuesday they were ready to fight under the banner of a U.S.-backed alliance of Kurds, Arabs and Syriac Christians making gains in the east of the country.
The Syrian Observatory for Human Rights quoted a statement from rebels in Idlib and Aleppo as saying at least 15 factions were now prepared to fight under the banner of the Syrian Democratic Forces.
The SDF -- which comprises a series of units under the Syrian Arab Coalition, The Syriac Military Council, Kurdish forces known as the People's Defense Units, or YPG, and the Women's Defense Units, or YPJ -- announced its formation in mid-October.
With support from U.S. airstrikes, the SDF earlier this month launched an offensive to clear Islamic State militants out of eastern Syria's al-Hasakah province and began making gains in areas around the provincial capital.
SOHR quoted the statement from the rebels in Idlib and Aleppo as saying they wished to work under the SDF banner due to "recent developments" in the country, including "victories achieved by SDF in al-Jazira area" -- referring to a former name for lands that now encompass al-Hasakah province.
The Syrian military, backed by Russian airstrikes, Lebanese Hezbollah fighters and Iranian troops, has since last month conducted a series of countrywide counter-attacks in a bid to regain territories lost earlier in the year, including in Idlib and Aleppo provinces.
The assaults came shortly after Russia intervened in Syria on Sept. 30 and began conducting airstrikes on behalf of President Bashar al-Assad, its regional ally.
An alliance of rebel groups in northwestern Syria -- including al-Qaida's Syrian affiliate, the Nusra Front -- had by April pushed the Syrian military out of all major urban strongholds in Idlib province, and by August, pro-Assad forces had been pushed west into Latakia province, heartland of Assad's ruling Alawite minority.
Among the rebel groups listed in Tuesday's statement was Ahrar ash-Sham, or the "Free Men of Syria." The group, estimated to comprise 10,000 to 20,000 fighters, is the lead cell in a rebel coalition known as the Islamic Front.
The United States has shown little interest in supporting Ahrar ash-Sham because of its cooperation with Nusra Front.
"They are in a gray zone, but in a civil war if you are not willing to talk to factions in the gray zone, you'll have precious few people to talk to," Robert S. Ford, a former United States ambassador to Syria now at the Middle East Institute, told The New York Times in August.
"I do not advocate giving any material support to Ahrar, much less lethal material assistance, but given their prominence in the northern and central fronts, they will have a big role in any peace talks, so we should find a channel to begin talking to them."