Iran has accused the US and Britain of being behind the assassination of an Iranian nuclear scientist this week in Tehran.

Iran's foreign ministry has sent a diplomatic letter to the US saying that it has "evidence and reliable information" that the CIA provided "guidance, support and planning" to assassins "directly involved" in Mostafa Ahmadi-Roshan's killing, the IRNA state news agency reported on Saturday.

Ahmadi-Roshan, a 32-year-old chemist, was killed in Wednesday morning traffic by motorcycle-borne assassins. It was the fifth time in two years that a scientist from the state nuclear programme had been targeted. Each time, the hit squad used a motorcycle.

His death has prompted calls in Iran for retaliation against those deemed responsible.

The US has denied any role in the assassination.

Iran delivered the letter to the Swiss embassy in Tehran, which looks after US interests in the country. The US has had no diplomatic relations with Iran since its 1979 revolution.

IRNA also reported that Iran delivered a letter to Britain accusing the UK of having an "obvious role" in the killing. It said that a series of assassinations began after British intelligence chief Sir John Sawers hinted in 2010 at intelligence operations against Iran.

Sawers has previously said intelligence-led operations were needed to make it more difficult for countries like Iran to develop nuclear weapons.

General Masoud Jazayeri, the spokesman for Iran's joint armed forces staff, said Tehran was "reviewing the punishment" of "behind-the-scene elements" involved in the assassination, IRNA reported.

"Iran's response will be a tormenting one for supporters of state terrorism," he said, without elaborating. "The enemies of the Iranian nation, especially the United States, Britain and the Zionist regime, or Israel, have to be held responsible for their activities."

Jazayeri also accused the International Atomic Energy Agency of being partly responsible, saying that the UN nuclear watchdog made public a list of Iranian nuclear scientists and officials that "has provided the possibility of their identification and targeting by spy networks".

The British Foreign Office has condemned the killing of civilians. Israeli officials have hinted at covert campaigns against Iran without directly admitting involvement.

The killing has sparked outrage in Iran, and state TV broadcast footage on Saturday of hundreds of students marching in Tehran to condemn Roshan's death and calling for the continuation of the country's controversial nuclear programme.

The US and its allies fear Iran is trying to develop nuclear weapons. Iran denies this, maintaining the programme is for peaceful purposes only.

The Republican presidential contender Rick Santorum said on Saturday that the US was wrong to condemn the killing.

The national security council spokesman Tommy Vietor said this week: "The United States had absolutely nothing to do with this. We strongly condemn all acts of violence, including acts of violence like this."

And Santorum, who is know for his hardline attitude towards Iran, said while on the campaign trail in South Carolina: "Our country condemned it. My feeling is we should have kept our mouth shut."