BEIRUT (AP) -- Activists say nearly 700 people have been killed in rebel infighting in opposition-held parts of northern Syria over the past nine days.

The fighting between the al-Qaida-linked Islamic State of Iraq and the Levant and an array of Islamist and more moderate rebel factions is the most serious violence within the armed opposition since the Syrian conflict began.

The British-based Syrian Observatory for Human Rights activist group said Sunday that at least 697 people have been killed since the clashes began Jan. 3. The toll includes 351 fighters from the Islamist and mainstream opposition brigades, 246 from the "Islamic State," and 100 civilians.

The al-Qaida-linked group has alienated other factions by using brutal tactics to implement its strict interpretation of Islamic law, and by kidnapping and killing of opponents.