The U.S. ambassador to the United Nations' main disarmament body walked out of its session on Tuesday to protest that Venezuela had taken the chair.
The move by Robert Wood, who insisted "a rogue state" was taking over, came shortly after Venezuelan Ambassador Jorge Valero began hosting a public plenary session of the Conference on Disarmament.
The Trump administration has been trying to put international pressure on Venezuelan President Nicolás Maduro's government. The U.S. and more than 50 other countries, mostly in Latin America and Europe, have recognized opposition leader Juan Guaidó as Venezuela's president, but Maduro remains in power.
Wood said that nothing that comes out of the current session will be legitimate, and said the United States will boycott Venezuela's four-week presidency starting this week.
He said a dozen members of the so-called Lima Group of countries from Latin America also decided not to take part.
The walkout and boycott appeared mostly to be political theatrics, however. Wood acknowledged that the conference "isn't doing very much right now."
It was far from clear whether the U.S. move would have an impact on a body that has struggled and failed to come to consensus on major issues in recent years.
Venezuela's turn as the conference's president follows a regular rotation by alphabetical order. The previous president was the United States.
Wood also led a boycott of Syria's presidency of the conference last year.