The US ambassador to the UN in Geneva said it was "a travesty" that Syria would head the Conference on Disarmament, leaving briefly when the Syrian representative took the floor.
"Syria's presence here is a travesty. This regime has committed countless crimes against its own people through the use of chemical weapons, and it is just unacceptable for them to be leading this body," Robert Wood told AFP just before the session began.
"They don't have the moral authority or the credibility to preside over this body, and we are going to make very clear during their four-week presidency that we find it abhorrent," he added.
Syria on Monday took over the rotating presidency of Conference on Disarmament (CD), according to a decades-old practice among the body's 65 members following the alphabetical order of country names in English.
During the first open session of Syria's presidency on Tuesday, the United States led a number of diplomats in protest.
Wood briefly left the room when Syria's ambassador Hussam Edin Aala opened the session, before returning to voice Washington's displeasure from the floor.
"Today marks a sad and shameful day in the history of this body," he told the assembly.
"Let me be clear: we cannot permit 'business as usual' in the CD while Syria presides over this body.
"During the next four weeks, we will be present in this hall to ensure that Syria is not able to advance initiatives that run counter to the interests of the United States, but we will fundamentally alter the nature of our presence in the plenaries," he said.
Following his statement, Wood moved in protest to a seat usually reserved for assistants.
Wood's words were echoed by a number of ambassadors from other countries, including Britain, Australia.
The French representative also said Syria "does not have the moral authority to lead this body."
