Western intelligence sources told Time magazine on Friday that Israel's Mossad is responsible for the latest assassination of an Iranian nuclear scientist.

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. Sources tell the magazine Israel was behind three previous assassinations of scientists.

Open gallery view The shrouded body of Mostafa Ahmadi-Roshan is seen prior to his burial in Tehran on January 13, 2012. Credit: AP

A senior Israeli official told is quoted in the report as saying "yeah, one more I don't feel sad for him."

On Saturday, Iranian state television said that Iran had evidence the United States was behind the latest assassination. 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.

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.

Also 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.

