The UN Security Council has blacklisted Syria's al-Nusra Front as an alias of al-Qaeda in Iraq, a decision that will subject the group to sanctions including an arms embargo, travel ban and assets freeze, diplomats said.

The US mission to the United Nations said none of the 15 council members objected to adding al-Nusra as an alias of al-Qaeda in Iraq on Thursday.

Al-Nusra, one of the most effective forces fighting President Bashar al-Assad, last month pledged allegiance to al-Qaeda
leader Ayman al-Zawahri.

The US State Department designated al-Nusra as a terrorist organisation in December 2012.

Experts have long said al-Nusra is receiving support from al-Qaeda-linked fighters in neighbouring Iraq.

The group claimed responsibility for deadly bombings in Damascus and Aleppo, and its fighters have joined other Syrian rebel brigades.

Syria had initially asked for al-Nusra to be designated a new terrorist group, but Britain and France countered with a proposal to instead list it as an alias of al-Qaeda because there were concerns about the Syrian evidence supporting its request, said diplomats, speaking on the condition of anonymity.

"There was quite a lot of suggestions that the information they got was from interrogation, which means torture in Syria," said a senior diplomat.

Either way the group was listed, it would still have been subjected to the same sanctions.

Videos of executions and torture by both sides of the conflict have become increasingly common in Syria.

Syria's conflict started more than two years ago with mainly peaceful demonstrations against Assad, but descended into a civil war in which the United Nations says at least 80,000 people have been killed.

The Syrian Observatory for Human Rights, a pro-opposition monitoring group based in the United Kingdom, said that more than 94,000 people had been killed.