Asia and Europe have vowed to support a cut in Iran's oil exports, though fear of self-inflicted economic pain is curbing enthusiasm for an embargo that Iran says will not halt its nuclear programme.

The Speaker of Iran's Parliament Ali Larijani said Iran's nuclear programme is also too strong to be derailed by assassinations of nuclear scientists, a day after the fourth such killing.

A newspaper close to the clerical establishment has called for retaliatory assassinations of Israeli officials, a former UN inspector said a new, almost bomb-proof plant could provide Iran enough enriched uranium for an atom bomb in just a year.

Iran denies all Western charges that it even wants nuclear weapons, have added to speculation that Israel and the US could resort to a military attack on the Islamic Republic, something an aide to Russian leader Vladimir Putin said was growing more likely.

After a motorcycle hit man blew up the 32-year-old engineer during the Tehran rush hour, many Iranians directed anger over the violence, and over painful economic sanctions, at the Western powers, which have hoped to turn popular sentiment against increasingly divided ruling elite.

Supreme Leader Ayatollah Ali Khamenei said that those behind yesterday's mystery killing would be punished.

Hossein Shariatmadari, who he appointed editor-in-chief of the Kayhan newspaper, wrote: "These corrupted people are easily identifiable and readily within our reach... Assassinations of the Zionist regime's military men and officials are very easy."

Kremlin Security Council head Nikolai Patrushev, close to Putin, was quoted blaming Israel, which says an Iranian bomb would threaten its existence, for pushing for war: "There is a likelihood of military escalation of the conflict, towards which Israel is pushing the Americans."

Former UN nuclear inspection chief Olli Heinonen said this week's announced start of uranium enrichment at a bunker complex could provide Iran with the ability to have enough such material for one nuclear bomb early next year - though it was not clear it would yet have the ability to build one.

Meanwhile, Senior UN nuclear officials are set to travel to Tehran later this month for a rare visit to discuss their growing concerns that Iran may be seeking to develop atomic arms capability.

A high-level team from the UN International Atomic Energy Agency (IAEA) is expected to visit Iran around 28 January even though the exact timing has not yet been finalised, two sources said, one of them suggesting it could also happen a day later.