Western and Arab nations sympathetic to the Syrian uprising have given full political recognition to the coalition of opposition groups led by Sunni Muslim cleric Mouaz Alkhatib.
An announcement by the American president, Barack Obama, followed similar announcements by France, Britain, Turkey and Gulf states. Mr Alkhatib was given an invitation on Wednesday to visit the United States.
However Washington and its allies also remain wary of Sunni Islamist fighters among the rebels, some of whom they say are connected to al Qaeda, Reuters reports.
The United States designated one powerful rebel group, the Jabhat al-Nusra brigade, as a terrorist organisation, a decision Mr Alkhatib said should be reversed.
At a meeting in the Moroccan city of Marrakech the "Friends of Syria" said President Bashar al-Assad had lost legitimacy and should stand aside to allow a sustainable political transition.
The Marrakech gathering brought together more than 100 countries, led by Western and Arab nations opposed to Mr Assad, but excluding Russia, China and Iran, which have backed the president or blocked efforts to tighten international pressure on him, Reuters reports.
Meanwhile, in the Syrian capital, Damascus, a car bomb has exploded at the main gate of the interior ministry, causing an unknown number of casualties.