Mohamed ElBaradei has not commented directly on the spying claims

The Washington Post newspaper said the US spied on Mr ElBaradei to see if his dealings with Tehran incriminated him.

The US opposes Mr ElBaradei's plans to seek a third term in office next year.

Mr ElBaradei has said he has no clear proof that Iran is making a nuclear bomb, contradicting US assertions that its arms programme calls for sanctions.

The US has repeatedly called for the International Atomic Energy Agency, headed by Mr ElBaradei, to refer Iran to the UN Security Council over its apparent efforts to conceal its nuclear weapons programme.

Mr ElBaradei says the "jury is still out" on whether Iran is trying to develop nuclear weapons or whether its programme serves a purely peaceful purpose, as Tehran maintains.

'Breaking the rules'

IAEA spokesman Mark Gwozdecky said the agency worked on "the assumption that one or more entities may be listening to our conversations".

"It's not how we would prefer to work, but it is the reality. At the end of the day, we have nothing to hide," he said.

Tehran has responded to the Washington Post reports by accusing the US of violating international law.

Iranian government spokesman Abdollah Ramazanzadeh said no secrets were passed in discussions with Mr ElBaradei, but added: "This is not the first time we have seen the US violate international rules."

White House spokesman Scott McClellan said he could not comment on the spying claims, but said he did not believe Mr ElBaradei was entitled to seek a third term in office.

Iraq argument

The Australian government has meanwhile said it has faith in Mr ElBaradei's ability to do his job, amid claims - also carried in the Washington Post - that the US was pushing for Australian Foreign Minister Alexander Downer to challenge him.

The Washington Post reported that Mr Downer was not willing to stand against Mr ElBaradei when the IAEA board elected a president next year.

The US has previously clashed with Mr ElBaradei over his assessment that Iraq did not possess weapons of mass destruction.

It has also been accused of spying on the UN's chief arms inspector, Hans Blix, in the run-up to the invasion of Iraq.