Al-Nusra Front fighters swept through a rebel-held town in northern Syria in a display of dominance,
arresting US-backed fighters and looting weapons stores belonging to the Free Syrian Army (FSA).
The fighters belonging to the al-Qaeda affiliate along with other armed groups 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.
The FSA's 13th Division said on Twitter on Sunday that al-Nusra Front fighters were going door to door in the town of Maarat Numan and arresting its cadres. Seven Division 13 fighters died in clashes the night before.
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The Syrian Observatory for Human Rights, a Britain-based monitoring group, said al-Nusra Front seized anti-tank missiles, armoured vehicles, a tank, and other arms from the division, which has received weapons, training, and money from the United States government.
It said al-Nusra Front and the Jund al-Aqsa group imprisoned 40 fighters from the FSA division.
Maarat al-Numan had a pre-war population of about 60,000 and saw some of the liveliest demonstrations calling for President Bashar al-Assad's fall in rebel-held areas over the past two weeks.
The protests were possible because the partial ceasefire brokered by the US and Russia brought relative peace to many beleaguered areas.
But armed fighters 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 northern Syria.
Last week, US Secretary of State John Kerry said that violence in Syria has been reduced by "80 to 90 percent" since the UN-sponsored "cessation of hostilities" came into effect on February 27.
Since the ceasefire, however, the Syrian government and its opponents have traded repeated accusations of breaches.
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