The UN nuclear watchdog has asked Iran to explain evidence that it has carried out experiments with highly advanced nuclear warhead design.

International Atomic Energy Agency (IAEA) believes Iranian scientists may have tested high explosive components of a device called "two-point implosion", the British newspaper the Guardian reported.

This technology would allow Iran capability to produce smaller and simpler warheads as this device is used to reduce the diameter of warheads and make it easier to mount it on a missile, the paper said.

Documents relating to experimentation using these device are part of the evidence of Iran's nuclear weaponisation gathered by the IAEA and presented to Tehran for its response.

"It is breathtaking that Iran could be working on this sort of material," the paper quoted a European government adviser on nuclear issues as saying.

A British nuclear expert James Acton said that Iran before perfecting step one is "going straight to step four or five...to start with more sophisticated design."

Another Western nuclear specialist said, "It raises a question who supplied this to them. Did A Q Khan has access to this or is it another player?"

The existence of "two-point implosion" device technology is officially secret in both the US and Britain, the Guardian said.

The strides made by Iran's nuclear programme come amid widespread fear among Western power that Iran is developing nuclear weapons, something which Tehran denies.

Iran and Western powers are currently engaged in negotiations how to procure nuclear fuel for civilian energy use by Tehran.

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