A ranking International Atomic Energy Agency official called Tehran's possession of a drawing showing how to make part of an atomic warhead "alarming" Thursday and said the onus is on Iran to prove it had not tried to develop nuclear arms, said diplomats attending a closed briefing.

The U.S. said the evidence detailed by IAEA Deputy Director General Olli Heinonen increased concerns that Tehran had tried to make such weapons. "Today's briefing showed ... strong reasons to suspect that Iran was working covertly and deceitfully at least until recently to build a bomb," Gregory L. Schulte, the chief U.S. delegate to the agency, told reporters.

Rejecting the allegation, Ali Ashgar Soltanieh, Schulte's Iranian counterpart, again dismissed the evidence as "baseless and fabricated documents and papers."

"The CIA has done a lousy job," he said.

Separately, a senior diplomat suggested the agency was not accepting as fact U.S. intelligence estimates that the Islamic Republic stopped active pursuit of nuclear weapons five years ago.

Queried on documents in the agency's possession possibly linked to research in such weapons and bearing dates into early 2004, he told The Associated Press that the IAEA was reserving its judgment on whether they indicated nuclear weapons work past 2003 until it finished its own investigations.

The documents, outlined in an IAEA report forwarded Monday to the U.N. Security Council and agency board members, are part of evidence provided by board member nations to the agency for its investigation into allegations that Iran used the cover of peaceful nuclear activities to conduct research and testing on a nuclear arms program.

One, dated January-February 2004 is linked to high explosives testing of the kind that can be used to detonate a nuclear device. Others, dated into January 2004 — and one as late as March 14 of that year — are part of purported evidence that Iran worked on designs of a missile re-entry vehicle that is normally a part of a nuclear delivery system.

The senior diplomat, who is familiar with agency attempts to investigate the nuclear weapons allegations, said the dates could mean nothing more than a review of activities that ended before 2004 but added the IAEA could make a final judgment only if Iran was forthcoming on requests to explain these and other documents. He demanded anonymity because he was not authorized to divulge confidential information.

A summarized U.S. National Intelligence Estimate, made public late last year, came to the conclusion that Tehran was conducting atomic weapons work but froze such activities in 2003. Other countries, however, believe such activities continued beyond that year, and any Iranian focus on nuclear weapons work in 2004 would at least indicate continued interest past the timeframe outlined in the U.S. intelligence estimate.

At the closed meeting Thursday, Heinonen said about 10 nations had provided intelligence and documentation meant to assist his team in investigating the allegations of hidden nuclear weapons work by Iran, said the diplomats. That marks the first time a precise number of countries was mentioned. The U.S. was the first country to share intelligence with the IAEA to support its allegations, and Tehran has depicted the probe as based on lies fabricated by Washington.

The diplomats quoted Heinonen as saying that Iran's possession of a drawing showing how to mold uranium metal into the shape of a warhead was "alarming" — even though it was not the ultimate key to making a nuclear weapon — because it raised questions about why a non-nuclear weapons state would want to have it.

He urged Iran to provide "plausible evidence" to back up its claims that its high-explosives testing — which fits the pattern used to detonate a nuclear payload — was for non-nuclear purposes.

The briefing followed up on Monday's IAEA report, which said Iran may be withholding information needed to establish whether it tried to make nuclear arms.

The report also said Iran remains defiant of the council's demands that it suspend uranium enrichment and has expanded its operational centrifuges — machines that churn out enriched uranium — by about 500 since the last IAEA report, in February.

The IAEA report noted Iran now had only 3,500 centrifuges and said the few advanced machines actually running were only in a testing phase. Still, a senior U.N. official said Iran's goal of 6,000 machines running by the summer was "pretty much plausible."

Uranium can be used as nuclear reactor fuel or as the core for atomic warheads, depending on the degree of enrichment. Iran says it is interested in enrichment only for its nuclear power program.