BEIRUT, Lebanon - Heavy fighting broke out Monday between rival jihadi groups in an oil-rich eastern Syrian province bordering Iraq, forcing many residents to flee, activists said.

The Britain-based Syrian Observatory for Human Rights said the fighting concentrated on the eastern parts of Deir el-Zour province. An activist based in the province who goes by the name of Abdul-Aziz Sheik said many tribesmen have joined the battle on the side of the Syrian al-Qaeda affiliate known as the Nusra Front, which is fighting an al-Qaeda breakaway group, the Islamic State of Iraq and the Levant.

The infighting comes ahead of a presidential election on June 3 that President Bashar al-Assad is expected to win. The vote gives him a mandate to continue his violent crackdown on rebels in the Syrian civil war.

In Deir el-Zour province, no side has made major gains since last week's capture of much of the western parts of the province by the Islamic State, according to the Observatory and Sheik.