Robert F. Kennedy Jr. claimed that the agency, Gavi, had “ignored the science” in immunizing children around the world.

The U.S. health secretary, Robert F. Kennedy Jr., who has moved to undermine public immunization programs in the United States, took his efforts global on Wednesday, accusing the leading international vaccine organization of having “ignored the science” in immunizing children around the world.

In confrontational remarks sent by video to global leaders who had gathered to support the organization, Gavi, Mr. Kennedy said the United States would withdraw its financial support for the group’s work purchasing vaccines for children in poor countries.

“When vaccine safety issues have come before Gavi, Gavi has treated them not as a patient health problem, but as a public relations problem,” Mr. Kennedy said in the address. He offered no evidence for the allegation.

His speech was the first indication that the Trump administration’s decision to end funding for Gavi may be motivated by mistrust of vaccines, in addition to a desire to reduce foreign aid.

In the video, which was played in the last minutes of the summit, Mr. Kennedy accused Gavi’s leaders of being selective in their use of science to support vaccine choices, and said that the United States would not deliver on a $1.2 billion pledge made by the Biden administration until the organization changed its processes.

“In its zeal to promote universal vaccination, it has neglected the key issue of vaccine safety,” he said. “I’ll tell you how to start taking vaccine safety seriously: Consider the best science available, even when the science contradicts established paradigms. Until that happens, the United States won’t contribute more to Gavi.”