At least 51 terrorists were killed as rival militant groups clashed in Syria on the border with Iraq on Thursday, a monitoring group said.

At least 51 terrorists were killed as rival militant groups clashed in Syria on the border with Iraq on Thursday, a monitoring group said.

The fighting erupted at dawn after an assault by the Islamic State of Iraq and the Levant on posts held by Al-Qaeda affiliate Al-Nusra Front and its allies.

The clashes centered around the border town of Albu Kamal, from which ISIL was expelled from earlier this year, said the opposing UK-based Syrian Observatory for Human Rights.

It said 51 combatants were killed.

The bodies of another 10 militants, apparently executed by ISIL, were found at two sites near Albu Kamal, the Observatory said, adding there were reports of other summary executions in the same area.

Earlier, the monitoring group's head Rami Abdel Rahman told AFP that ISIL was advancing in Albu Kamal and had taken control of several districts previously held by Al-Nusra and other terrorist brigades.

The clashes prompted soldiers on the Iraqi side of the border to reinforce their positions.