The Iranian Foreign Ministry sent a protest letter to the U.S. government in which it said that the U.S. Central Intelligence Agency (CIA) is behind the recent terrorist attack which led to the assassination of an Iranian nuclear official, the local English language Tehran Times daily reported on Sunday.

The 32-year-old, Mostafa Ahmadi-Roshan, the deputy chief of the commercial section of Iran's Natanz nuclear enrichment site, was killed in a car bomb attack in northern Tehran on Wednesday.

The letter addressed to the U.S. government, read, "according to authentic documents and reliable information, the assassination plot was directed, supported, and planned by the CIA and was carried out with the direct involvement of the agents affiliated with this agency, and the (U.S.) government is directly responsible for it and should be answerable based on international regulations," said the daily.

The protest note was delivered to Swiss Ambassador to Tehran Livia Leu Agosti on Jan. 14. The Swiss embassy in Tehran represents the U.S. interests in Iran since Tehran and Washington severed diplomatic relations in 1980.

Also, Iran's Foreign Ministry said that it has sent another protest note to the UK government over the assassination of the Iranian nuclear scientist Majid Shahriari, in November 2010, according to Tehran Times.

In the letter, it was said that the assassination of Shahriari followed the remarks of the MI6 chief in October 2010 calling for intelligence-led operations against Iran over its controversial nuclear program.

U.S. State Secretary Hillary Clinton on Wednesday denied that the United States was involved in the assassination of an Iranian nuclear staff.

"I want to categorically deny any United States involvement in any kind of act of violence inside Iran," Clinton said at a press conference at the State Department alongside visiting Qatari Prime Minister and Foreign Minister Sheikh Hamad bin Jassim al-Thani.

Several Iranian nuclear scientists have been assassinated in the recent years and Iran accused the United States, Britain and Israel of being behind the attacks.