Fighters from the al-Qaeda affiliated al-Nusra Front group have driven the rebels of the Western-backed Syrian Revolutionary Front from their bastion in the northwestern province of Idlib after 24 hours of fighting.
The defeat, in which some of the SRF fighters deserted and joined al-Nusra, will be a blow to US efforts to create and train a moderate rebel force as a counterweight to both self-declared jihadists and the Syrian government.
Al-Nusra captured Deir Sinbel, and also now controls most of the other towns and villages in the Jabal al-Zawiya area, the Syrian Observatory for Human Rights said.
"[The al-Nusra Front] seized SRF arms and tanks, and some of the rebels swore allegiance to al-Nusra, while others fled," the Britain-based organisation said.
Western-backed rebel groups in Syria are coming under growing pressure as Assad forces continue to advance in the country’s west and south, while the self-declared jihadists make territorial gains in the east and north.
The so-called moderate rebels complain of lack of credible military support from US and other allies.
The SRF, backed by the West and various Arab countries, had earlier vowed to establish a democratic state in when and if President Bashar al-Assad was toppled.
The group was formed at the end of 2013 as an alliance of moderate rebels organisations.
It was hoped that its leader, Jamaal Maarouf, would spearhead the formation of a nationwide force capable of challenging, not only the the self-declared jihadists, but also the regime.
'Bastard, like Baghdadi'
With the loss of its bastion in Idlib, the SRF is now left with only a presence in southern Syria.
On Friday, activists published a video showing Maarouf addressing the al-Nusra leader Abu Mohammad al-Jolani.
"You have tarnished the name of Islam, and you have tarnished religion. Why do you fight us? Go away, fight the regime!" he said. "You are nothing; you are just like Baghdadi ... you bastard."
Maarouf was referring to the chief of the Islamic State in Iraq and the Levant, Abu Bakr al-Baghdadi, whose group controls large swathes of the country’s east and is accused of committing horrendous crimes there.
Al-Nusra hit back on Twitter, accusing Maarouf of "corruption" and of "straying from the path of the revolution".
Idlib province was among the first areas to fall from government control soon after the outbreak of a revolt against Assad's in 2011.
Source: Al Jazeera and agencies