VIENNA (REUTERS) - The United States (US) said on Wednesday it was "deeply troubled" by Iran's plans to start a reactor in 2014 that could yield nuclear bomb material while failing to give United Nations (UN) inspectors necessary design information about the plant.

The comments by a US envoy to a board meeting of the UN's International Atomic Energy Agency (IAEA) highlighted deepening Western concern about the heavy water reactor that Iran is building near the town of Arak.

Iran hit back, saying the IAEA had had continuous access to Arak and that the US, which accuses Tehran of seeking nuclear weapons capability, could not be trusted after going to war in Iraq over reports of weapons of mass destruction that were never found.

"We will not yield to pressure, sanctions, threats of attack," Ambassador Ali Asghar Soltanieh told reporters, underlining Tehran's determination to press ahead with its nuclear programme despite international demands to curb it.

Tension over Iran's nuclear course is rising with talks between Tehran and six powers stalled.

Israel, widely assumed to be the Middle East's only nuclear-armed state, sees Iran as its greatest threat and has threatened to bomb its nuclear sites if diplomacy and sanctions fail to restrain Tehran.

Iran says the Arak plant will make isotopes for medical and agricultural use. But analysts say this type of facility can also produce plutonium for weapons if the spent fuel is reprocessed - something Iran says it has no intention of doing.

Tasked with ensuring that nuclear material is not diverted for military purposes, the IAEA says Iran must urgently give it design data about Arak to allow it to monitor the site properly.

"We are deeply troubled that Iran claims that the IR-40 heavy water reactor at Arak could be commissioned as soon as early 2014, but still refuses to provide the requisite design information," Mr Joseph Macmanus, the US ambassador to the IAEA, told the 35-nation Board of Governors.

He cited IAEA rules that a member state must inform the Vienna-based UN agency about a nuclear plant, and give design details, as soon as it has decided to build one. Iran says it must only do so 180 days before bringing atom fuel to the plant.

"Iran's refusal to fulfil this basic obligation must necessarily cause one to ask whether Iran is again pursuing covert nuclear activities," Mr Macmanus said.

CHINA AND RUSSIA PRESS IRAN

IAEA chief inspector Herman Nackaerts told the board that Iran last provided design information about Arak more than six years ago, when it was at an early stage of construction.

It was "sketchy or incomplete and additional information is now required", he said, according to one diplomat.

The West suspects Iran is seeking the capability to develop nuclear weapons behind the facade of an atomic energy programme.

Iran says its nuclear work is a bid to generate electricity and also to make progress in other areas of scientific research.

It denies accusations it is seeking to develop atomic weapons.

Western worries about Iran are focused largely on uranium enrichment plants at Natanz and Fordow, as such material refined to a high level can provide the fissile core of an atomic bomb.

But diplomats and experts say Arak could offer Iran another route to nuclear bombs, if it decided to build them.

Experts say Arak could produce enough plutonium for one bomb per year, if Iran decided to pursue such weapons of mass destruction. But it would first have to build a facility to chemically separate the material from the spent fuel.

"We don't have any plan for reprocessing," Mr Soltanieh said.

Israel has twice before bombed suspected reactor construction sites in the Middle East - in Iraq in 1981 and in Syria in 2007 - underlining its determination not to allow regional foes to acquire the means to assemble nuclear weapons.

To signal big power unity on Iran, China and Russia joined four Western powers in pressing Iran at the IAEA meeting to cooperate with a stalled investigation by the UN nuclear agency into suspected atomic weapons research by Tehran.

In a joint statement, the six powers said they were "deeply concerned" about Iran's nuclear activities.