Syrian rebels with the jihadist al-Nusra Front, labelled a terrorist outfit by the US government, have executed three soldiers loyal to President Bashar al-Assad in the city of Deir Ezzor, according to the British-based Syrian Observatory for Human Rights.

In a video released by the group and posted on YouTube, the three soldiers, seated in front of a black flag bearing the Muslim profession of faith, give their identities to masked men branding Kalashnikovs.

The soldiers reveal themselves as Alawites, an offshoot of the Shiite faith to which president Bashar al-Assad and senior officials belong. The graphic footage then shows the bodies of the men lying in a ditch.

The US has denounced the al-Nusra front, in the vanguard of the rebel fighters, as being terrorists and an appendix of al-Qaida.

Footage of two pro-Assad Alawites being beheaded with a machete in the street by Western-backed Sunni rebels, one of them a child, started circulating on YouTube and raised questions over the legitimacy of Western intervention in the Syrian civil war.

A report by the Quilliam Foundation, a counterterrorism policy institute based in London, showed that al-Nusra had become the most effective of the factions fighting the Assad regime and now can count on 5,000 fighters. The group is led by veterans of the Iraqi insurgency.

"The civil war in Syria is a gift from the sky for al-Nusra; they are coasting off its energy," the lead author of the report, Noman Benotman, told CNN.

The report states that al-Nusra is a Syrian offshoot of al-Qaida in Iraq (AQI), the outfit founded by the Abu Masab al-Zarqawi, who was killed in 2006 by a US missile strike.

The US State Department has warned that AQI is trying to "hijack the struggles of the Syrian people for its own malign purposes".

"AQI emir Abu Du'a is in control of both AQI and al-Nusra. Du'a also issues strategic guidance to al-Nusra's emir, Abu Muhammad al-Jawlani, and tasked him to begin operations in Syria," the State Department said.

The most important figure in al-Nusra is Jawlani, allegedly a former associate of Zarqawi.

Benotman, himself a former prominent Jihadist who knew Osama bin Laden and Zawahiri, claims that the purpose of al-Nusra is the setting up of an Islamic state in Syria. The group is very selective about recruiting new members, according to the Quilliam study.

Al-Nusra has claimed responsibility for nearly 600 attacks since the beginning of the conflict, including 40 suicide attacks and bombings in major cities.

The report reveals also that the group carried out executions of media professionals an military officers and members of the feared pro-Assad Shabiha militia.

Fighters from the Islamist Syrian rebel group Jabhat al-Nusra clean their weapons in Aleppo
Fighters from the Islamist Syrian rebel group Jabhat al-Nusra clean their weapons in Aleppo (Reuters)