The U.N. nuclear watchdog wants to send a high-level mission to Iran to address mounting concerns it may be seeking to design atom bombs, its head said on Thursday, and Tehran was expected to face censure at a meeting of agency member states.

An International Atomic Energy Agency report last week assessing that Iran has been conducting research and experiments geared to developing a nuclear weapons capability has stoked tensions in the Middle East and heightened Western pressure for harsher sanctions against the Islamic Republic.

IAEA chief Yukiya Amano announced his proposal at a meeting of its 35-nation governing board, where six major powers were set to close ranks to express deep concern about Iran's activities and call on it to open up fully to U.N. inspectors.

Vienna-based Western diplomats said the powers had agreed compromise language for a draft resolution, to be put to governors for approval by Friday, after Western states and Russia overcame divisions sparked by the IAEA's report on Iran.

But the resolution will not satisfy those in the West and in Israel, Iran's arch-enemy, who had hoped Amano's document would trigger concrete international action to rein in Tehran.

Amano said he had written to the head of Iran's Atomic Energy Organisation, Fereydoun Abbasi-Davani, earlier this month to suggest the visit, which would air issues raised by the hard-hitting IAEA report on Iran.

Preventing the proliferation of nuclear weapons is one of the IAEA's core responsibilities, the veteran Japanese diplomat told the closed meeting, according to a copy of his speech.

Throughout the past three years, we have obtained additional information which gives us a fuller picture of Iran's nuclear programme and increases our concerns about possible military dimensions, Amano said.

The information indicates that Iran has carried out activities relevant to the development of a nuclear explosive device, he said, in his toughest public statement so far on Iran's contested nuclear programme.

Iran denies that it is seeking atomic weapons, dismissing intelligence information in the IAEA report as fabricated, and accusing the U.N. watchdog of pro-Western bias.

Amano said he hoped a suitable date could be agreed soon for his team's visit to Iran, which permits IAEA inspections of declared nuclear sites but since 2008 has stonewalled an agency investigation into alleged studies applicable to atomic bombs.

WEST SEEKS MORE IRAN PRESSURE

It is essential that any such mission should be well planned and that it should address the issues contained in my report, Amano said.

I ask Iran to engage substantively with the agency without delay and provide the requested clarifications regarding possible military dimensions to its nuclear programme.

The fact that the six big powers were ironing out an IAEA resolution will be welcomed in the West after Amano's report prompted Russia to complain that it was politicised and dimmed chances of a negotiated solution to the Iran nuclear dispute.

Moscow's stance exposed big power divisions over how to best to resolve it: Western states seized on the IAEA report to try to step up pressure on Tehran in the form of farther-reaching economic sanctions, which Russia and China oppose.

It (the IAEA resolution) will maintain pressure on Iran, one Western diplomat said. He and others said they were waiting for Beijing to formally approve the text before putting it to the board meeting, which runs through Friday.

In November 2009, IAEA governors including Russia and China rebuked Iran for building a uranium enrichment plant in secret. Iran rejected that vote as intimidation.

The latest draft text -- expected to be co-sponsored by the United States, Britain, France, Germany, Russia and China -- would stop short of taking concrete actions such as reporting Iran once again to the U.N. Security Council.

There has been concern that if the powers cannot settle their differences over how to nudge Iran into serious nuclear negotiations, then Israel, which feels endangered by Iranian nuclear aspirations, will attack it.

Israel is widely believed to have the Middle East's only nuclear arsenal to deter numerically superior enemies, but has never confirmed or denied it.

Russia has significant trade ties with Iran and also built its first nuclear power plant, launched at Bushehr earlier this year. China is a major importer of Iranian oil.

(Editing by Mark Heinrich)