TEHRAN (Reuters) - The U.N. atomic watchdog’s top investigator began talks with Iranian officials on Monday to press them to explain Western intelligence which suggested Iran had covertly studied how to design nuclear bombs.

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Iran’s official IRNA news agency said Olli Heinonen, who arrived in Tehran on Monday, met Javad Vaeedi, deputy head of the Islamic Republic’s Supreme National Security Council, behind closed doors.

Monday’s discussions lasted for about three hours and will resume on Tuesday, the ISNA news agency said. IRNA said Heinonen would leave Tehran on Wednesday after two days of meetings and did not plan to visit any of Iran’s nuclear facilities.

Ali Asghar Soltanieh, Iran’s representative at the International Atomic Energy Agency (IAEA), said Heinonen’s visit was intended to advance cooperation between Iran and the IAEA, the U.N. body investigating Iran’s disputed nuclear ambitions.

“Such talks ... display Iran’s policy and determination to carry on its technical and expertise cooperation with the IAEA,” Soltanieh told the semi-official Fars news agency.

Last week, U.S. President George W. Bush and British Prime Minister Gordon Brown pledged a united effort to prevent Iran from developing a nuclear bomb, possibly by expanding sanctions.

Iran says its nuclear program is a peaceful drive to produce electricity so that the world’s fourth-largest crude exporter can sell more of its oil and gas abroad. It has been hit by three rounds of limited U.N. sanctions since 2006 because of its inadequate cooperation with IAEA investigations.

Heinonen raised a diplomatic stir in February with a presentation that indicated links in Iran between projects to process uranium, test explosives and modify a missile cone in a way suitable for a nuclear warhead.

Iran has dismissed the intelligence as baseless, forged or irrelevant. But the IAEA wants substantive explanations to enable it to wind up a long inquiry into Iran’s secretive quest for nuclear power.

IAEA Director Mohamed ElBaradei has said the world “needs to make sure Iran did not have a weapons program”.

An Iranian official quoted by IRNA said Tehran had presented its “evaluation” of the issue to the U.N. agency but did not rule out discussing the matter with Heinonen.

“Since there are differences between Iran and the agency (over the Western intelligence) the meeting ... will deal with finding a solution to this problem,” ISNA quoted an official from Iran’s Atomic Energy Organisation as saying.

IAEA officials stress that the intelligence details about weapons studies, many of them from a laptop computer spirited out of Iran by a defector in 2004 and handed to the United States, remain unverified but warrant thorough investigation.

World powers are considering improving a package of trade and other incentives for Iran if it stops enriching uranium, which can be used as nuclear fuel or provide material for bombs.

But Iran has refused to halt its nuclear drive and says it is working on its own proposals to help defuse the row.