Iran's interior minister vowed on Monday to take revenge on Israel over the mysterious bomb attack which killed a nuclear physics professor last week.

Iranian officials have blamed an exiled opposition group, accusing it of acting on behalf of Israel and the U.S. A U.S. official rejected as "absurd" the Iranian accusations that the United States had a role in the slaying.

"Charges of U.S. involvement are absurd," said a U.S. State Department official.

Israel did not comment.

Interior Minister Mostafa Mohammad Najjar on Monday promised revenge on Zionists but did not elaborate.

The 50-year-old Tehran University professor, Masoud Ali Mohammadi, was killed when a bomb-rigged motorcycle exploded outside his home January 12.

It remains unclear why the researcher with no prominent political voice, no published work with military relevance and no declared links to Iran's nuclear program was targeted.