The US National Intelligence Estimate on Iran -- released in December -- said Tehran had been working on the development of nuclear weapons, but abandoned the programme in 2003.

But Britain's ambassador to the IAEA, Simon Smith, said the material presented to the board of governors on Monday contained information about possible weapons work beyond that date.

"Certainly some of the dates that we were talking about, or that the secretariat was presenting in there, went beyond 2003," Smith said.

He was talking after being briefed by the International Atomic Energy Agency's head of safeguards, Olli Heinonen, about its latest report on Iran's disputed nuclear programme.

The briefing focused on allegations of Iran's involvement in weaponisation studies.

Diplomats attending the briefing said the material presented to the board of governors had infuriated Iranian ambassador Ali Asghar Soltanieh.

"He seemed angry," one western diplomat said, speaking on condition of anonymity.

The diplomat added that Soltanieh was so enraged and went on at such length that IAEA deputy director general Heinonen, who chaired the meeting, had to intervene twice to ask him to make his point.

Tehran has refused to address the weaponisation studies issue, dismissing such allegations as "baseless" and saying the intelligence used to back them up was "fabricated."

Nevertheless, the IAEA insists the allegations have to be cleared up and explained fully if it is to be able to determine the full nature of Iran's nuclear programme.

In the IAEA report released on Friday, the watchdog described the issue as "a matter of serious concern and critical to an assessment of a possible military dimension to Iran's nuclear programme."

The board was presented with material "from multiple sources" suggesting "detailed work put into the designing of the warhead, studying how that warhead would perform, how it would be detonated and how it would be fitted to a Shahab-3 missile," Smith said.

The material was "serious and substantial," the ambassador continued.

And it underlined the IAEA secretariat's concern that the deficit of confidence in Iran, rather than being reduced, "if anything, is getting deeper," he said.

In an interview with AFP on Sunday, Soltanieh complained that Tehran was not given enough time to respond to the new material, since it was based on intelligence the IAEA had only been authorised to show to Iran on February 15.

But Smith dismissed such claims.

"These are not new questions that Iran has been given half an hour to answer. They've been around a very long time," the ambassador said.

And Iran's answers remain "incomplete, inadequate and evasive," he said.

Quizzed by reporters as he left the meeting, Soltanieh acknowledged he was "upset" and had "forcefully" warned the other board members that the allegations -- because they were connected with activities not directly of a nuclear nature -- were outside the mandate of the IAEA.

Soltanieh dismissed the documents and slides shown to the meeting as amateurish and the intelligence as "lousy", saying the whole thing could easily have been drawn up "by any undergraduate."