“We are working with European allies on a brand-new launch to decriminalize homosexuality,’’ Mr. Grenell told NBC, despite other issues that divide relations between Europe and the United States. He also said he had spoken to senators supportive of using American foreign aid as part of the effort, although he declined in the interview to specify which senators.

A number of American allies are among the nations that outlaw and severely penalize homosexuality, including Saudi Arabia, and it remained unclear how the effort led by Mr. Grenell would persuade the Saudis or others to end deep-seated discrimination.

Mr. Grenell told NBC that the activists, from Turkey, Ukraine, Bulgaria and other European countries, had joined him and an Iranian expatriate around a large table to discuss the effort. The ambassador said it would require “71 different strategies.”

Also unclear was whether the decriminalization effort was, in fact, new. A State Department spokesman in Washington, Robert Palladino, queried after the NBC report on the effort was broadcast, said: “This really is not a big policy departure. This is longstanding and it’s bipartisan.”

American-led diplomacy to promote gay and transgender rights internationally expanded under President Barack Obama, well before the Trump administration took office. The United Nations Human Rights Council passed its first resolution establishing L.G.B.T. rights as human rights in 2014.

David Pressman, a former American diplomat who is a partner at the Boies Schiller Flexner law firm, who helped lead the Obama administration’s international L.G.B.T. rights strategy, said that despite his contacts among groups who work on such issues, he first heard about Mr. Grenell’s undertaking from news reports.

“What’s surprising is that no one who works these issues within the U.S. government appears to be aware of this effort,’’ Mr. Pressman said. “No one in the groups who have been engaged on these issues appears to be aware of this effort.”