WASHINGTON: Secretary of State Hillary Rodham Clinton pulled out of a weeklong trip to the Arab world because of a stomach virus, officials said Monday, as the Obama administration declared a Syrian rebel group with alleged ties to al Qaeda as a terrorist organisation.  

The announcement was one of several Clinton planned to bring with her to North Africa and the Middle East as part of a US effort to enhance cooperation with moderates in the Syrian coalition fighting President Bashar Assad's regime, and to isolate extremists in their ranks.

Clinton's deputy, Deputy Secretary of State William Burns, will take her place in Morocco, Tunisia and the United Arab Emirates.

''Since she's still under the weather, we'll be staying put this week instead of heading to North Africa and the Middle East as originally planned,'' Clinton's spokesman, Philippe Reines, said.

On the first stop, in the Moroccan city of Marrakech on Wednesday, Burns likely will recognise Syria's new opposition coalition as the legitimate representative of the Syrian people, officials said. Clinton had been expected to make the declaration, which is designed to reward anti-Assad leaders for making their movement more inclusive and facilitate greater American assistance.

Earlier Monday, the administration took action against the rebel militia Jabhat al-Nusra, which has claimed responsibility for suicide bombings on Syrian government targets and raised fears of growing Islamic extremism among the opposition.

By branding the group a terrorist organization, the US government is freezing any assets Jabhat al-Nusra holds in the United States and barring Americans from doing business with the group.

The action hasn't been announced officially, but was included in the Federal Register on Monday. In the notice, the State Department described the group as part of al Qaeda in Iraq.