Al Qaeda militants swept through a rebel-held town in northern Syria in a display of dominance Sunday, arresting U.S.-backed fighters and looting weapons stores belonging to the Free Syrian Army.

The militants belonging to the Al Qaeda-affiliated Nusra Front along with allied jihadists have been moving to exert their authority over rebel-held areas in Idlib province since a partial ceasefire to the country's five-year conflict took effect two weeks ago, extinguishing patriotic demonstrations and sidelining nationalist militias.

The FSA's 13th Division said on Twitter Sunday that Nusra fighters were going door to door in the town of Maarat Numan and arresting its cadres after Nusra, alongside fighters from the Jund al-Aqsa faction, seized Division 13 posts the night before.

Seven Division 13 fighters died in the clashes.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said Nusra seized anti-tank missiles, armored vehicles, a tank, and other arms from the division, which has received weapons, training, and money from the U.S. government. It said Nusra and Jund al-Aqsa detained 40 fighters from the division.

Maarat Numan had a prewar population approaching 60,000 and saw some of the liveliest demonstrations calling for President Bashar Assad's fall in rebel-held areas over the last two weeks as the partial ceasefire brokered by the United States and Russia brought relative peace to many beleaguered areas.

But jihadist militants have repeatedly tried to suppress the demonstrations in Idlib province, where they maintain a strong presence. The challenges have threatened to fracture the array of forces allied to prevent Syrian government forces from retaking north Syria.

The Nusra Front and Jund al-Aqsa suppressed a demonstration in Idlib city last week, arresting several demonstrators and allegedly replacing the tricolored flag of the Syrian uprising with the black flag of the Al Qaeda movement, according to opposition accounts. Another hard-line Jihadist group, Ahrar al-Sham, sided publicly with the demonstrators in a carefully worded statement that did not name any responsible parties.

Nusra supporters stormed another demonstration in Maarat Numan Friday, again carrying black banners, but were drowned out by the protesters. Rumors circulated Saturday that Division 13, which maintained a presence in the town, then tried to push Nusra out. By Sunday morning, it was clear that Nusra had overpowered their rivals, instead.

The Nusra Front arrested U.S.-supplied fighters belonging to FSA's 30th division last summer and seized their weapons in a major embarrassment to the U.S. government's train-and-equip program which was meant to support carefully vetted "moderate" rebels.