Iran vowed Monday to take revenge on Israel and the United States for the slaying last week of a physics professor in a mysterious bomb attack, the official news agency reported.

Iranian officials have blamed the bombing on an exiled opposition group known as the People's Mujahedeen, accusing it of acting on behalf of Israel and the U.S. The armed opposition group and Washington have denied involvement, while Israel has not commented.

A week after Masoud Ali Mohammadi's death, it remains unclear why the 50-year-old Tehran University professor would have been a target for assassins who left a bomb-rigged motorcycle outside his home on Jan. 12. Ali Mohammadi had no prominent political voice, no published work with military relevance and no declared links to Iran's nuclear program, though his work included some aspects of nuclear theory.

"Rest assured that Iran will take revenge for the blood of martyr Ali Mohammadi from you," Interior Minister Mostafa Mohammad Najjar said, addressing Israel and the U.S.

"Such a blind move, which is the result of acts by the Mossad, the CIA and enemies of the Islamic Republic of Iran's system, shows their weakness," Najjar said. His comments were reported by the official IRNA news agency.

President Mahmoud Ahmadinejad has described the assassination as having been carried out in a "Zionist style," saying it showed their "grudge" against the Iranian nation.

Key figures among both Iran's pro-reform opposition and hard-line government supporters have condemned the professor's killing.

Ali Mohammadi had few apparent links outside the academic community.

He was not known to have any key roles in the opposition movement, although his name appeared on a university petition pledging support for pro-reform candidate Mir Hossein Mousavi before June's disputed presidential election.

Mousavi claimed he was deprived of the presidency through fraud, triggering months of street protests and a harsh crackdown by the authorities.

Ali Mohammadi's assassination took place at a time of high tension in Iran, as authorities grapple with how to contain a resilient opposition movement that has moved from just challenging the election result to confronting Iran's clerical leadership.

On Monday, Iran's judiciary put five people on trial over their alleged roles in anti-government protests in December that sparked the worst street violence in months. If found guilty, they could face the death penalty.

The five, who were not identified, have been accused of cooperating with the People's Mujahedeen, the same group Iran is blaming for the bombing that killed the professor. A broadcast on state TV from inside the courtroom showed the defendants, but their faces were not visible.

At least eight people died in the clashes late last month between security forces and opposition supporters across Iran, including a nephew of Mousavi, the opposition leader. It was the worst bloodshed since the height of the unrest immediately after President Mahmoud Ahmadinejad's disputed re-election.

A prosecutor read out a lengthy indictment against the five, accusing them of a crime against Islam and the state known as moharebeh, which is punishable by death. The word is Farsi for defying God.

The June election has polarized Iran, with moderates withdrawing support for or being dismissed by the hard-line government and others resigning in protest. Among those resigning was an Iranian diplomat in Norway.

On Monday, Foreign Minister Manouchehr Mottaki urged the diplomat, Mohammad Reza Heidari, to get back to service. Mottaki said his resignation was not acceptable and that "he should continue his job either in Norway or the ministry."

Heidari told Voice of America's Farsi service Sunday that he resigned to protest the bloody crackdown against the opposition. VOA said he has defected to Norway.