The slaying is sure to add to tension with the West, as Iran moves ahead with an atomic program that four rounds of UN sanctions have failed to slow.

The target, identified in Iranian media reports as 35-year-old Darioush Rezaei, was a physics professor whose area of expertise was neutron transport, which lies at the heart of nuclear chain reactions in reactors and bombs. One of the earlier assassinations, in November, killed a man with the same specialization.

Several news reports, including by the semi-official ISNA news agency, linked him to the country's nuclear program.

Two attackers on motorbikes approached Rezaei's car as he was driving up to his home in northeastern Tehran with his wife and young daughter, said the news website asriran.com.

The assailants fired five shots before fleeing, and Rezaei was hit in the neck and hand, asriran reported.

His wife was wounded in the attack and rushed to a hospital for treatment, said the semi-official Mehr news agency, quoting a police officer.

The governor of Tehran, Morteza Tamaddon, said intelligence and security agencies have begun an investigation but no one has been arrested, the official IRNA news agency reported.

Despite the UN and other sanctions, Iran has steadily moved ahead with its uranium enrichment work, the central aspect of its nuclear program and the process that is of deepest concern to the West because it can be used both to produce reactor fuel and material for nuclear warheads.

Iran insists it is only after reactor fuel, but the UN's nuclear watchdog agency has accused Iran of stalling its investigation into the work for years.

In November, a pair of back-to-back bomb attacks in different parts of the capital killed one nuclear scientist and wounded another. President Mahmoud Ahmadinejad blamed the US and Israel.

In those attacks, assailants on motorcycles attached magnetized bombs to the cars of two scientists as they drove to work. They detonated seconds later.

The man who survived that attack, Fereidoun Abbasi, is on a list of figures suspected of links to secret nuclear activities in a 2007 UN sanctions resolution, which put a travel ban and asset freeze on those listed.

Abbasi has since been named one of Iran's vice presidents and head of its nuclear agency.

The scientist killed in that attack had the same area of expertise as Rezaei.

At least two other Iranian nuclear scientists have been killed in recent years.