The US has designated the al-Nusra Front, an important fighting force in the Syrian's opposition's efforts, as a foreign terrorist organisation, saying it was trying to hijack the uprising on behalf of al-Qaeda in Iraq (AQI).
Officials in Washington said on Tuesday that the group had claimed responsibility for carrying out nearly 600 attacks in major cities that have killed numerous innocent Syrians during the 20-month uprising against President Bashar al-Assad.
"Through these attacks, al-Nusra has sought to portray itself as part of the legitimate Syrian opposition while it is, in fact, an attempt by AQI to hijack the struggles of the Syrian people for its own malign purposes," US State Department spokeswoman Victoria Nuland, said in a statement.
Designating al-Nusra as a terrorist group means US authorities can freeze any assets the group or its members have in US jurisdictions. It also prohibits US citizens from giving it any material support.
Al Jazeera's Hashem Ahelbarra said that Syria's opposition coalition views the US designation as "ill-timed" and unwise.
"They basically believe that not all al-Nusra Front fighters are necessarily al-Qaeda fighters and they do have the backing of Syrians," he said from from Morocco's Marrakesh, where a Friends of Syria meeting is to take place on Wednesday.
"They basically said that they would like the Americans to look into the bigger picture ... that the Americans should stop thinking about the world from that perspective, which is getting rid of al-Qaeda, wherever al-Qaeda is manifesting itself."
The US Treasury, in a parallel move, also slapped sanctions on two armed groups working for the Assad government, including the Shabiha which is accused of atrocities against civilians.
US officials conceded that the action was unlikely to immediately curtail al-Nusra's activities, but said it was an important signal both to the Syrian opposition and its foreign supporters, particularly in the Gulf, that al-Nusra and similar groups cannot play a part in Syria's eventual political transition.
"I think that other nations that are involved in helping the armed opposition will now take more seriously our concerns about the Nusra front and its expanding influence," one senior US official told reporters in a briefing on the move.
"It is important for countries to understand what al-Nusra is and what it represents."
Tuesday's action came as US officials prepared to attend the Friends of Syria meeting in Marrakesh to discuss the Syria crisis, as rebels push forward on the battlefield and move to unify the political opposition.
Al-Nusra, one of the most efficient fighting groups in Syria, is not part of a new military unified rebel command elected in Turkey over the weekend.
Some other groups value it for its fighting prowess while others accuse it of indiscriminate tactics, such as poorly placed car bombs that have harmed civilians, and summary executions.
A high-level official in the Syrian Muslim Brotherhood said on Tuesday the US had made a "very wrong" decision.
"The designation is very wrong and too hasty. I think it is too early to categorise people inside Syria this way, considering the chaos and the grey atmosphere in the country," Farouk Tayfour, deputy leader of the group, told Reuters news agency.
Tayfour, a key figure in the Brotherhood, the best organised group among the civilian opposition to President Bashar al-Assad, said the decision would lose the US popularity in Syria because the Nusra Front is liked by Syrians bearing the brunt of Assad's crackdown on the 20-month-old revolt.
"They are seen as (a group that) can be relied on to defend the country and the civilians against the regular army and Assad's gangs," Tayfour said.
"It could be that a person from al-Nusra in some village carried arms and grouped people to defend against Assad's militiamen and these people are now designated as Nusra but after the regime falls they may leave the whole ideology and not consider it its own," he added.