The US has recognised the new Syrian opposition coalition as "the legitimate representative" of the country's people, just a few hours after labelling an important Syrian rebel group as terrorists for their links with al-Qaeda.

President Barack Obama told ABC that the National Coalition for Syrian Revolutionary and Opposition Forces "is now inclusive enough, is reflective and representative enough of the Syrian population," to assume the important role.

However US Secretary of State Hillary Clinton had earlier designated the jihadist al-Nusra Front, which has played a prominent role in the rebellion against Bashar al-Assad's regime, as a terrorist organisation.

Attempting to explain the distinction, Obama told ABC: "Not everybody who's participating on the ground in fighting Assad is people who we are comfortable with.

"There are some who, I think, have adopted an extremist agenda, an anti-US agenda, and we are going to make clear to distinguish between those elements."

As a consequence of the designation, al-Nusra's assets in the US have been frozen and Americans are prohibited to deal with the group, which boasts between six and ten thousand fighters.

The double diplomatic move is set to marginalise Islamic extremists in the rebellion and in the possible post-Assad Syria. However it also risks backfiring, distancing rebels fighting on the ground from the coalition.

The National Coalition for Syrian Revolutionary and Opposition Forces, which has already been recognised by the UK, France Turkey and six Gulf states, has been labelled by al-Nusra as a "conspiratorial project" imposed by the West.

The National Coalition was formed after the previous dominant opposition group - the Syrian National Council - failed to establish itself as an effective and popular alternative to Assad, because of its inability to understand the needs of the rebels fighting on the ground.

Although they are aware that al-Nusra wants to turn Syrian in an Islamic state, the rest of the Syrian rebel fighters, who are mainly Sunni Muslims, see the group as a precious ally dedicated to the common cause of toppling Assad.

"They have their own leaders and their own structure. They fight side by side with the Free Syrian Army, we have only seen good things from them," Abdul Jabbar al-Oquaidi, a senior member of the opposition coalition single military command, told The Times.

Following the US designation, another 29 Syrian opposition groups signed a petition in support of al-Nusra's "heroes" and called for mass demonstrations.

"These are the men for the people of Syria, these are the heroes who belong to us in religion, in blood and in revolution," the petition reads.

Syrian rebels have also often blamed the West for lack of practical support in their fight against Assad, while al-Nusra has been able to play a highly effective role in the rebellion thanks to the support it receives from Saudi Arabia and Qatar.

The US designations also plays into the hands of Assad's rhetoric, as the regime has maintained it is fighting terrorists since the very beginning of the rebellion.

Seventy countries including the US and the UK are to meet in Marrakech, Morocco imminently to discuss the situation in Syria, together with the Syrian National Coalition.

After having recognised the Syrian National Coalition, the UK took a more cautious stance towards al-Nusra.

"We need to look carefully at the case for proscribing al-Nusra and understand better the dynamics between this group and more moderate opposition groups in Syria. We want to support the moderate groups and ensure extremists don't divert the popular uprising in Syria to their own ends," a UK foreign office spokesperson told IBTimes UK.