Jihadists from the Islamic State of Iraq and the Levant (ISIL) have executed three officers of the Western and Arab-backed rebel Free Syrian Army, a monitor said Saturday.

The bullet-riddled bodies of the three were found on Friday, two days after suspected Islamist militants kidnapped them in the oil-rich eastern province of Deir Ezzor, said the Syrian Observatory for Human Rights.

ISIL, which grew from the ranks of al-Qaida before splitting with the global terror network, is active in both Syria and neighboring Iraq and seeks to set up an Islamic state that straddles both countries.

But the radical group has faced a major backlash from other rebels fighting to topple the Syrian regime and from Al-Qaeda's affiliate in Syria, Al-Nusra Front, amid accusations of atrocities in Syria's three-year war.

The FSA this month called for help from "friendly and brotherly Arab nations" to fight ISIL in Deir Ezzor.

Rebels in Syria, including Al-Nusra Front militants, have been battling ISIL since the start of the year, in fighting that has reportedly killed more than 6,000 people.

The clashes have mostly raged in the north and east of the country, and on Friday ISIL seized control of the areas of Hafez and Muhassen in Deir Ezzor.

The FSA officers executed by the jihadists had been kidnapped in Hafez, the Britain-based Observatory said.

ISIL has been battling Iraqi government forces since June 9, when in launched a lightening offensive in the north of the country.