The full reasons the sources have gone silent are not known. But current and former officials also said the exposure of sources inside the United States has also complicated matters.

This year, the identity of an F.B.I. informant, Stefan Halper, became public after House lawmakers sought information on him and the White House allowed the information to be shared. Mr. Halper, an American academic based in Britain, had been sent to talk to Trump campaign advisers who were under F.B.I. scrutiny for their ties to Russia.

Current American officials said there is no direct evidence that the exposure of Mr. Halper has been cited by overseas informants as a source of concern.

But the officials said that some allies have cited the exposure of the informant and other intelligence leaks in curbing some of the intelligence they share.