Bomb kills key Iranian scientist Mostafa Ahmadi Roshanan

Mostafa Ahmadi Roshan was inside the car with two others when the bomb exploded (Reuters)

An Iranian nuclear scientist was killed by a bomb in his car in what appears to be the latest covert assassination to hinder the country's nuclear programme.

Two men on a motorbike attached a magnetic bomb to the car of Mostafa Ahmadi Roshanan, a university professor working at a key nuclear plant.

The attack in Tehran was similar to earlier killings of scientists working on the Iranian nuclear programme. It is certain to amplify authorities' claims of clandestine operations by Western powers and their allies.

Professor Roshan was a chemistry expert and a director of the Natanz uranium enrichment facility in central Iran.

Iran's state news agency IRNA said he had organisational links" to Iran's nuclear agency, which suggests a direct role in key aspects of the programme.

Natanz is Iran's main enrichment site, but officials claimed earlier this week that they are expanding some operations to an underground site south of Tehran with more advanced equipment.

The West is pressuring Iran to halt uranium enrichment, a key element of the nuclear programme suspected to be producing atomic weapons. Uranium enriched to low levels can be used as nuclear fuel but at higher levels, it can be used as material for a nuclear warhead.

Iran denies it is trying to make nuclear weapons, saying its programme is for peaceful purposes only.

Tehran has accused Israel's Mossad, the CIA and Britain's MI6 of engaging in an underground "terrorism" campaign against nuclear-related targets, including at least three killings since early 2010 and the release of a malicious computer virus known at Stuxnet in 2010 that temporarily disrupted controls of some centrifuges - a key component in nuclear fuel production. All three countries have denied the Iranian accusations.

Israeli officials, however, have hinted about covert campaigns against Iran without directly admitting involvement.

Iran's first vice president Mohammad Reza Rahimi said Israeli agents were behind the attack, but cannot "prevent progress".

A similar bomb explosion two years ago killed Tehran University professor Masoud Ali Mohammadi.

In November 2010, a pair of back-to-back bomb attacks in different parts of the capital killed another nuclear scientist and wounded one more.

And in July 2011, motorcycle-riding gunmen killed Darioush Rezaeinejad, an electronics student. Other reports identified him as a scientist involved in suspected Iranian attempts to make nuclear weapons.