American espionage in Iraq, under cover of United Nations weapons inspections, went far beyond the search for banned arms and was carried out without the knowledge of the UN leadership, it was reported yesterday.

An investigation by the Washington Post found that CIA engineers working as UN technicians installed antennae in equipment belonging to the UN Special Commission (Unscom) to eavesdrop on the Iraqi military. When British intelligence asked what was going on, the operation was denied, the report said.

US government officials refused to comment on the report yesterday.

In response to newspaper allegations of espionage in January, the US conceded that it had deployed eavesdropping equipment in an operation codenamed Shake the Tree, but insisted that it was done at the invitation of Unscom with the sole aim of foiling Saddam Hussein's attempts to conceal weapons of mass destruction.

But according to yesterday's report, quoting unnamed US sources, the "remote monitoring system" Unscom used to relay video pictures of suspected weapons sites to inspectors in Baghdad was secretly used to intercept communications between Iraqi commanders and military units.

Richard Butler, the Australian diplomat who runs Unscom, was reportedly kept in the dark about the CIA operation, as was his predecessor, Rolf Ekeus.

But the Washington Post quoted "sources in Washington" as saying that the CIA notified Charles Duelfer, a US official who served as deputy to both Mr Ekeus and Mr Butler, to ensure that Unscom inspectors in Iraq did not interfere with the operation.

On hearing of the Washington Post story, Mr Butler was reported to have exclaimed to a colleague: "If this stuff turns out to be true then Rolf Ekeus and I have been played for suckers, haven't we?"

Ewen Buchanan, spokesman for Unscom, said yesterday that Mr Butler had made no official response to the Washington Post allegations. But he questioned whether the alleged eavesdropping would have been possible. "I don't know if it was technically feasible," he said.

"Those repeater stations [used in the Unscom video-monitoring programme] are out in the middle of nowhere. They're there for anyone to go and tinker with. Wouldn't the Iraqis have been able to tell what was going on?" he asked.

The UN secretary-general, Kofi Annan, said he "personally had no direct knowledge" of the allegations.

In a book due to be published next month, a former US Unscom inspector, Scott Ritter, is expected to confirm that CIA agents infiltrated the UN inspection teams. Mr Ritter resigned from Unscom last year, complaining that the US was undermining its work by ordering inspections to be reined in to avoid military confrontations at politically inconvenient moments.

Iraqi officials said the report vindicated their claims that Unscom was a front for US and British spies.

According to yesterday's report, Unscom technicians occasionally noted "burst transmissions" but were unable to identify their source. But an Iranian spy in Baghdad was more successful and signalled back to Tehran a message saying that the US was running an electronic espionage operation within Unscom.

British intelligence intercepted the Iranian message, but when it asked the US National Security Agency for an explanation in May 1997, the NSA gave none."We don't tell the British everything, even if they are our closest intelligence ally," the Post quoted one US official as saying.

Unscom weapons inspections collapsed in December, when US and British planes carried out Operation Desert Fox. By then, according to yesterday's report, the clandestine eavesdropping network had already been abandoned.