Iran is on the brink of developing an atomic missile, weapons inspectors of the UN's nuclear watchdog have warned.

A report by the International Atomic Energy Agency (IAEA) said Iranian scientists had made at least 22 kg of enriched uranium with at least 20 percent purity, the Daily Telegraph reported Tuesday.

Experts estimate that 20 kg of uranium is the minimum required to arm a warhead. The uranium would still need to have its purity raised to 90 percent but that is a relatively easy process.

The agency's report comes in spite of the recent imposition at the UN of a fresh round of sanctions against Iran and will heighten fears of an Israeli attack on Iran's nuclear plants.

The prospect of an attack had receded only recently with American assurances that Tehran was more than a year away from acquiring a bomb.

The Vienna-based watchdog said Tehran had maintained its absolute defiance of international pressure to curb its nuclear programme despite the imposition of harsh sanctions in May.

The IAEA has grown increasingly alarmed at Iran's behaviour. The latest report, which will be presented to the agency's governors next week, lambasted Tehran on a series of fronts.

The country's refusal to answer questions on its attempts to make a nuclear warhead that could be fitted on to its most advanced missiles was denounced as a violation of sanctions.

The agency also rebuked the Iran government for its repeated failure to co-operate with weapons inspections designed to ensure that material was held securely at Iranian plants.