The US ambassador to the UN in Geneva said on Tuesday 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 said just before the session began.
He also said that the US was planning to walk out and "will be taking other actions that you will see over the course of Syria's presidency, so please stay tuned".
Mr Wood said that Washington did not plan a boycott of the presidency but wanted to hold Damascus to account for its use of chemical weapons.
On Monday, the US led protests as Syria took over the rotating presidency of the Conference on Disarmament at a time when Damascus is widely accused of using chemical weapons.
"Monday, May 28 will be one of the darkest days in the history of the Conference on Disarmament with Syria beginning its four-week presidency", tweeted US ambassador to the CD Robert Wood.
The CD chair rotates alphabetically every four weeks and Syria's turn came around on Monday, although the conference will meet in full session only from Tuesday.
"The Damascus regime has neither the credibility nor moral authority to preside over the CD," Mr Wood said.
"The international community must not be silent.
"A regime that has used CW [chemical weapons] against its own people has no place presiding over the CD's work."
Britain also protested against the move.
"The UK deplores the fact that Syria will assume the presidency of the Conference on Disarmament, given the regime's consistent and flagrant disregard of international non-proliferation and disarmament norms and agreements," British ambassador Matthew Rowland said.
But he pointed out that all CD members, including Syria, had to agree to change the rotating system.
After hundreds of people died in chemical attacks near Damascus in 2013, a deal with Russia was struck to rid Syria of chemical weapons, staving off US air strikes.
A suspected chlorine and sarin attack in the Syrian town of Douma on April 7 this year triggered punitive missile strikes against alleged chemical weapons sites in Syria by the US, Britain and France.
Other permanent missions to the Geneva-based CD support the US and Britain and would protest against Syria's lack of "political legitimacy" and "moral authority" at the conference, a diplomatic source said.
Syria's role would not be allowed to damage the proper working of the CD, the source added.
Some ambassadors were not expected to lead their delegations at the conference in a sign of protest.
UN secretary general Antonio Guterres, who put his own disarmament proposals to the CD last week, said he had "no power" over the rotation system.
He said he hoped Syria's presidency would not have a "negative impact" on the important work of the conference.
The CD is a multilateral disarmament forum that meets in three sessions a year in Geneva. It negotiates arms control and disarmament accords and focuses on the cessation of the nuclear arms race.