The UN nuclear watchdog has only limited knowledge of the Stuxnet computer worm, the agency chief was quoted as saying, after reports that the cyber attack had slowed Iran's nuclear programme.

'(The) IAEA's knowledge on this Stuxnet worm is limited,' International Atomic Energy Agency chief Yukiya Amano was quoted by the Press Trust of India news agency as saying during a conference in the Indian financial capital Mumbai.

An IAEA spokesman in Vienna confirmed Amano's statement.

Iran's centrifuges producing enriched uranium have been plagued by breakdowns since a rapid expansion of the process in 2007-08. Security experts have speculated its Natanz enrichment plant have been targeted in a state-backed attack using Stuxnet.

Israel this month revised its evaluation of Iran's nuclear progress, with the outgoing spy chief saying Tehran would not be able to build a bomb for at least four years 'because of measures that have been deployed against them'.

The New York Times last week reported Israel had tested the Stuxnet worm in a heavily guarded nuclear complex as part of a joint effort with the US to undermine Tehran's nuclear ambitions.

Western leaders suspect Iran's declared civilian nuclear energy programme is a cover to develop weapons capability but Tehran says it is aimed only at producing electricity. – Reuters