US-backed rebels on Sunday accused Al-Qaeda-linked fighters of storming their headquarters in northwestern Syria and kidnapping their commander and dozens of other combatants.
In an online statement, Jaish al-Tahrir (Liberation Army) said its commander, Mohammad al-Ghabi, was abducted from his father's home in the town of Kafranbel by Al-Nusra Front jihadists on Saturday evening.
It said Al-Nusra fighters also "kidnapped more than 40 members of Jaish al-Tahrir" and stole weapons from other bases and checkpoints set up in northwestern Syria.
"We call on Al-Nusra Front to immediately release our commander and all the others who were kidnapped," the statement said, urging other Islamist groups to put pressure on the jihadists.
The Syrian Observatory for Human Rights confirmed the incident and said the US had supported Jaish al-Tahrir with weapons and even salaries in the past.
Al-Nusra has attacked several US-backed groups in northwestern Idlib province - most recently raiding the warehouses of Division 13 in the town of Maarat al-Numan in March.
And in the summer of 2015, it kidnapped several members of US-trained rebel group Division 30.
Idlib province is controlled by the Army of Conquest, an alliance of Islamist and rebel fighters led by Al-Nusra and independent Salafist group Ahrar al-Sham.
Residents of towns like Maarat al-Numan and Kafranbel have demonstrated against Al-Nusra's jihadist ideology.
Syria's five-year conflict began with anti-government demonstrations before turning into a complex war increasingly dominated by extremist groups.
More than 280,000 people have been killed and millions have been forced to flee their homes since March 2011.
AFP contributed to this report.