Iran nuclear arms research "serious concern": IAEA

VIENNA (Reuters) - Iran's alleged research into designing nuclear warheads remains a matter of serious concern and needs "substantive explanations," the U.N. nuclear watchdog said on Monday.

The International Atomic Energy Agency (IAEA) also said in its latest report on Iran that Tehran had 3,500 uranium enrichment centrifuges working at its Natanz underground nuclear facility, a slightly higher number than earlier this year.

The agency said it had not been given access to Iranian nuclear-related sites that it asked to see in April.

The IAEA has been pressing Tehran for answers after Western intelligence alleged that Iran had covertly studied how to design atomic bombs. Iran has dismissed the intelligence as baseless, forged or irrelevant.

"Substantive explanations are required from Iran to support its statements on the alleged studies and on other information with a possible military dimension," the agency said in its report.

"We have not got substantive answers and we could have gotten those earlier," a senior U.N. official said. "It's up to Iran (now)."

In a report in February, the IAEA said Iran had 3,000 older P-1 centrifuges running at very low capacity at the Natanz enrichment plant. A small number of "new generation" centrifuges, supposedly able to enrich 2-3 times as fast, were being tested in the above-ground pilot wing there.

Iran announced in April it had begun installing 6,000 new centrifuges at Natanz.

But later that month, IAEA chief Mohamed ElBaradei said between 3,300 and 3,400 centrifuges of the 1970s vintage P-1 type were operational at Natanz.

The IAEA said it asked Iran in April to provide access to locations related to the manufacture of uranium centrifuges, research and development of uranium enrichment, and uranium mining and milling.

"To date, Iran has not agreed to the agency's request," the report said.