Activists say rebel infighting in Syria has killed 45 opposition fighters over the past two days.

The Britain-based Syrian Observatory for Human Rights says the fighting flared up in eastern Deir el-Zour province on Sunday and continued into Monday.

The Observatory says the clashes pitted al-Qaida's affiliate the Nusra Front against the al-Qaida breakaway Islamic State of Iraq and the Levant.

The two jihadi groups were allies but had a falling out earlier this year and have since intermittently clashed in some of the fiercest rebel infighting in the Syrian civil war.

The Observatory says a month of infighting in Dier el-Zour alone has killed nearly 300 fighters and displaced 100,000 civilians.

It has also weakened the Syrian opposition's resolve to overthrow President Bashar Assad.