The United Nations nuclear watchdog wants to send a high-level mission to Iran to address mounting concerns the country may be seeking to design atomic bombs, its head said on Thursday.

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 punitive sanctions against the Islamic Republic.

IAEA chief Yukiya Amano announced his proposal at a meeting of the Vienna-based agency's 35-nation governing board, where six major powers were expected to close ranks to increase diplomatic pressure on 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-door 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 disputed 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.

Vienna-based Western diplomats said six world powers were close to finalising an agreement on a draft resolution at the two-day IAEA meeting expressing concern about Iran's activities and calling on it to cooperate fully with the IAEA.

The fact that the six major powers were set to agree on a joint text will be welcomed in the West after the IAEA 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 resolution before officially submitting it to the board meetingm, which runs through Friday.

But the draft board 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 the nuclear aspirations of its arch-enemy, 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.

The United States and its allies faced a dilemma ahead of this week's IAEA governors meeting: press for a strongly worded resolution and risk Russian and Chinese opposition, or accept a weaker text in order to preserve big-power unity.

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

(Editing by Mark Heinrich)