Iran has stymied a UN inquiry into whether it researched ways to make a nuclear bomb, the International Atomic Energy Agency (IAEA) said yesterday, and Britain said it would push hard for tougher sanctions on Iran.

A confidential IAEA report said Iran had raised the number of centrifuges enriching uranium by 500 to 3,820 since May and was testing an advanced model able to refine nuclear fuel 2-3 times faster, in continuing defiance of UN resolutions.

But a senior UN official familiar with IAEA findings said Iran seemed at least two years away from enriching enough uranium for an atomic weapon, if it eventually chose to do so.

"On the issue of possible military dimensions to Iran's nuclear programme, we have arrived at a gridlock. Without Iran's assistance and cooperation, we cannot move forward," said a second senior UN official.

Iran blamed the IAEA for the impasse. A senior Iranian official, who declined to be named, called on the IAEA to change its approach and work in a "legal and logical" manner.

The US called on Iran to shelve enrichment or face the possibility of more UN sanctions, adding to relatively modest punitive measures Tehran has shrugged off. Britain went further, accusing Iran of showing "contempt for the IAEA by continuing to refuse to respond" to IAEA chief Mohamed ElBaradei's serious concerns about research with possible military nuclear dimensions.

"We will therefore push hard for further UN sanctions in the coming weeks," a British Foreign Office statement said.

The report will be debated by the IAEA's 35-nation board of governors at a meeting starting on September 22, with the possibility that Western powers might seek a resolution against Iran.

Iran had stockpiled 480 kilos of low-enriched uranium so far, the report by the Vienna-based IAEA said.