Iran's unwillingness to explain alleged nuclear weapons-related projects only reinforces suspicions about past efforts to develop a nuclear bomb, the United States said at a meeting of the UN nuclear watchdog on Wednesday.

The US Ambassador to the International Atomic Energy Agency (IAEA), Gregory Schulte, told the organization's Board of Governors that unanswered questions 'strongly suggest Iran has undertaken a significant state-sponsored effort to develop nuclear weapons.'

IAEA chief Mohamed ElBaradei had said Monday it was 'regrettable' that the IAEA had not made the expected progress in clarifying possible military projects concerning missiles, high explosives testing and nuclear material production.

Iran would be served best by admitting its past nuclear weapons work and allowing the IAEA to verify that it has been halted, Schulte told the 35-nation board.

Like his European colleagues, he spoke in favour of a negotiated solution to the Iranian nuclear issue.

Zalmay Khalilzad, the US ambassador to the UN, said on Tuesday in New York that EU chief diplomat Javier Solana would go to Tehran on June 14 to deliver an upgraded incentive package to Iran as a new effort by the West to persuade Iran to suspend its uranium enrichment programme.

'We hope this contact will be possible without further delay and that Iran will respond positively to this substantial offer,' the French Ambassador to the IAEA, Francois-Xavier Deniau, told the board on behalf of Britain, France and Germany.

Sources in Brussels confirmed that Solana's visit to Iran was imminent, but his press office declined to confirm the date.

Together with the US, Russia and China, Britain, France and Germany have updated a package from 2006, offering cooperation in the nuclear field as well as economic, political and security-related incentives.

Most countries represented on the IAEA board called on Iran on Wednesday to heed the UN Security Council's demands for halting its uranium enrichment programme, diplomats said.

Iran was expected to deliver a response at the IAEA Board of Governors on Thursday morning.