BEIRUT — Fighting between an al-Qaida-linked group and a loose alliance of more moderate and ultraconservative rebel brigades has killed nearly 500 people over the past week in northern Syria, an activist group said Friday.

The rebel-on-rebel violence that broke out a week ago has spread across four provinces in opposition-held parts of the north in what amounts to the most serious infighting among opponents of President Bashar Assad since the country’s conflict began in March 2011.

The clashes pit fighters from a variety of Islamist and moderate factions against the Islamic State of Iraq and the Levant, an extremist group that has alienated many Syrians over the past several months by using brutal tactics to implement its strict interpretation of Islamic law.

The Britain-based Syrian Observatory for Human Rights said Friday that at least 482 people have been killed in the fighting. It said 240 of the dead were rebel fighters, while another 157 were from the “Islamic State.” The remaining 85 were civilians, the Observatory said.

The consortium of ultraconservative rebels and more moderate brigades have made headway against fighters from the “Islamic State” in several areas of the provinces of Aleppo, Idlib, Hama and Raqqa — although the al-Qaida-linked group has managed to regroup and curb some of its losses.

The Islamic State of Iraq and the Levant and another al-Qaida linked group known as Jabhat al-Nusra, or the Nusra Front, initially joined forces with moderate rebels fighting to oust Assad in a conflict that began as a popular uprising but morphed into a civil war.

The extremists proved well-organized and efficient fighters, giving the ragtag rebels a boost.

But the brutality of the “Islamic State” has turned other factions against it, leading to some of the worst infighting of the conflict.