Iran's alleged research into nuclear warheads remains a matter of serious concern and Tehran should provide more information on its missile-related activities, the U.N. nuclear watchdog said on Monday.

The International Atomic Energy Agency (IAEA) also said, in its latest report on Iran, that Tehran was holding back information on high-explosives testing relating to its nuclear programme.

It said Tehran had 3,500 uranium enrichment centrifuges working at its Natanz nuclear facility, slightly more than earlier this year, and a few more advanced centrifuges were also being tested.

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

Iran's research into "high explosives testing and the missile re-entry vehicle project remained a matter of serious concern", said the report, which will be passed on to the United Nations Security Council.

"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, though it added that it had not detected any actual use of nuclear material in connection with the alleged studies.

"We have not got substantive answers and we could have gotten those earlier," a senior U.N. official said.

The next step is up to Tehran and to the IAEA board of governors, which meets next week in Vienna, he said. "They (Iran) know what we need, everything is listed here that we need, the questions are clear...we need answers."

Ali Asghar Soltanieh, Iran's ambassador to the agency, said the IAEA report showed that Tehran's nuclear programme was peaceful, Iran's semi-official Fars News Agency reported.

The report "shows Iran's entire nuclear activities are peaceful," Fars quoted him as saying. "Once more it has been explicitly underlined that there has been absolutely no evidence regarding the diversion of Iran's nuclear activities or materials toward military purposes," he said.

IRAN 'BLOCKING' IAEA

Gregory Schulte, the U.S. envoy to the IAEA, said Iran was blocking the IAEA's efforts to investigate indications that it had engaged in studies, engineering work, and procurement relevant to building nuclear weapons.

"The report shows in great detail how much Iran needs to explain, and how little it has," he said.

"Altogether this report is a clear vote that Iran could have done more, but that it didn't," said a European diplomat.

The report said Iran had not given the IAEA all the necessary information or access to documents and individuals.

"The agency is of the view that Iran may have additional information, in particular on high explosives testing and missile-related activities, which could shed more light on the nature of these alleged studies and which Iran should share with the agency," the report said.

Iran, the world's fourth largest oil exporter, maintains that its nuclear programme is directed solely at the peaceful generation of electricity and rejects Western assertions that it is secretly pursuing nuclear weapons.

Iran has been the subject of three United Nations sanctions resolutions since 2006, all demanding that it cease its nuclear enrichment activities, which it has refused to do.

The five permanent U.N. Security Council members plus Germany are offering Iran a package of incentives to give up its uranium enrichment, so far without success.

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 few faster "new generation" centrifuges 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 as a transparency measure.

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