Iranian state television said on Saturday that Iran had evidence the United States was behind the latest assassination of one of its nuclear scientists.
In the fifth attack of its kind in two years, a magnetic bomb was attached to the door of 32-year-old Mostafa Ahmadi-Roshan's car during the Wednesday morning rush-hour in Tehran. His driver was also killed.
The United States has denied involvement in the killing and condemned it. Israel has declined to comment.
"We have reliable documents and evidence that this terrorist act was planned, guided and supported by the CIA," the Iranian foreign ministry said in a letter handed to the Swiss ambassador in Tehran, state TV reported.
"The documents clearly show that this terrorist act was carried out with the direct involvement of CIA-linked agents."
The Swiss Embassy has represented U.S. interests in Iran since Iran and the U.S. cut diplomatic ties shortly after the 1979 Islamic revolution.
State TV said a "letter of condemnation" had also been sent to the British government, saying the killing of Iranian nuclear scientists had "started exactly after the British official John Sawers declared the beginning of intelligence operations against Iran".
In 2010, chief of the British Secret Intelligence Service Sawers said one of the agency's roles was to investigate efforts by states to build nuclear weapons in violation of their
international legal obligations and identify ways to slow down their access to vital materials and technology.
Iran has urged the UN Security Council and Secretary-General Ban Ki-moon to condemn the latest killing, which Iran says is aimed at undermining its nuclear work, which the West and Israel say is aimed at building bombs. Ian says its nuclear program is purely civilian.
Tension has mounted between Iran and the West as the United States and European Union prepare measures aimed at imposing sanctions on the Iran's oil exports, its economic lifeblood.
The United States and Israel have not ruled out military action if diplomacy fails to resolve the nuclear dispute.
On Saturday, the Wall Street Journal reported that the U.S. has stepped up contingency planning in case Israel launches a military strike on Iran's nuclear facilities.
According to the report, U.S. defense officials are becoming increasingly concerned that Israel is preparing to carry out such a strike.
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