Associated Press
This photo provided by an anti-government activist group Edlib News Network (ENN), which has been authenticated based on its contents and other AP reporting, show Syrian rebels standing next to a tank that was captured from the Syrian government forces at the al-Dahman checkpoint, in southern Idlib, northern Syria, Wednesday, July 9, 2014.

BEIRUT — Fighters of an al-Qaida affiliate seized a northern border town in Syria from rival rebels, activists reported Tuesday, as new infighting threatened opposition gains.

The Nusra Front captured the town of Haram in northern Idlib province, said Rami Abdurrahman of the Britain-based Syrian Observatory. The town was seized after weeks of skirmishes between the Nusra Front and their one-time allies among the Western-backed rebel groups, who once united over seeking the overthrow of President Bashar Assad.

A spokesman for some Western-backed groups, Hussam al-Marie, confirmed the infighting. Another rebel group, the Hazzm Movement, said earlier this week that they would no longer work with the Nusra Front.

The infighting apparently began after Nusra Front fighters left a front threatened by government forces near the northern city of Aleppo. It later worsened after rebels accused Nusra fighters of not helping them hold a town on a strategic highway route in central Syria. Later, Nusra fighters clashed with a rebel group near the northern town of Darkoush, al-Marie said.

If fighting spreads, it may threaten rebel gains in Syria. Rebel forces already are badly weakened by government victories and infighting that began last winter against militants of an extremist group now called the "Islamic State."

The Islamic State group now dominates the arc of the Euphrates River from near the Turkish border in Syria deep into neighboring Iraq.

Although the Nusra Front is an al-Qaida affiliate, other Syrian rebels fought alongside their fighters, arguing that it was dominated by Syrian fighters — unlike the foreigners of the harsh "Islamic State" group. They also said that Nusra was not imposing its ultra-conservative agenda.

Nusra was one of the chief groups that fought against the Islamic State group.

Abdurrahman, who obtains his information from activists in Syria, said Nusra was trying to carve out its own stronghold in the Idlib province on the Turkish border. He said they seized the city of Jisr al-Shaghour earlier this month and other towns along the border route.

"It's clear from the clashes in the area — there is a project in Nusra," he said. "They are seizing towns and areas to be connected geographically."