The US administration has listened in on telephone calls between Dr Mohamed ElBaradei and Iranian diplomats, according to a US newspaper report.
It did so apparently in an attempt to gather information it could use to force Dr ElBaradei out of his job as head of the UN nuclear watchdog agency, the Washington Postreported.
"The intercepted calls have not produced any evidence of nefarious conduct by Dr ElBaradei," the Washington Postsaid, quoting three unnamed US officials who had read the transcripts.
"Some people think he sounds way too soft on the Iranians, but that's about it," one official was quoted as saying.
The US wants the UN International Atomic Energy Agency, which Dr ElBaradei heads, to report Iran to the UN Security Council for possible sanctions over what Washington says is a covert nuclear weapons programme.
However, Dr El Baradei says the "jury is still out" on whether Tehran's program is peaceful or not.
Washington's favourite replacement for Dr ElBaradei is the Australian Foreign Minister, Mr Alexander Downer, but he has been unwilling to challenge the IAEA chief, the newspaper reported.