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Fars News Agency, Associated Press
This undated photo released by Iranian Fars News Agency, claims to show Mostafa Ahmadi Roshan, who they say was killed in a bomb blast in Tehran, Iran, on Wednesday, Jan. 11, 2012, next to his son. Two assailants on a motorcycle attached a magnetic bomb to the car of an Iranian university professor working at a key nuclear facility, killing him and his driver Wednesday, reports said. The slayings suggest a widening covert effort to set back Iran's atomic program. The blast killed Mostafa Ahmadi Roshan, a chemistry expert and a director of the Natanz uranium enrichment facility in central Iran, state TV reported.

TEHRAN, Iran — A hard-line Iranian newspaper called Thursday for retaliation against Israel, a day after the mysterious killing of a nuclear scientist in Tehran with a magnetic bomb attached to his car. Iran's top leader blamed Israel and the U.S.

Provocative hints from Israel reinforced the perception that the killing was part of an organized and clandestine campaign to set back Iran's nuclear ambitions, which the U.S. and its allies suspect are aimed at producing weapons. Iran says the program is for peaceful purposes only.

Iran's nuclear confrontation with the West had already been escalating in the weeks before the killing, with the U.S. tightening sanctions against Tehran, and Iranian officials warning that they would shut a waterway vital to global oil shipping in response.

The Wednesday assassination of Mostafa Ahmadi Roshan — at least the fourth targeted hit against a member of Iran's nuclear brain trust in two years — has heightened tensions even further.

Iran's Supreme Leader Ayatollah Ali Khamenei blamed both Israel and the U.S. In a message read on Iranian state TV, he said the killing was carried out "with design or coordination of the CIA and the Mossad," Israel's spy agency. He pledged that Iran would punish those responsible.

A column in the Kayhan newspaper by chief editor Hossein Shariatmadari asked why Iran did not avenge Roshan, a chemistry expert and a director of the Natanz uranium enrichment facility, by striking Israel.

"Israeli military chief Lt. Gen. Benny Gantz in his recent remarks spoke about damaging Iran's nuclear program," he wrote. "Assassinations of Israeli military and officials are easily possible."

The day before the attack, Gantz was quoted as telling a parliamentary panel that 2012 would be a "critical year" for Iran — in part because of "things that happen to it unnaturally."

Tehran quickly blamed Israeli-linked agents backed by the U.S. and Britain. Secretary of State Hillary Rodham Clinton denied any U.S. role in the slaying, and the Obama administration condemned the attack. "I want to categorically deny any United States involvement in any kind of act of violence inside Iran," she said.

Israeli officials, in contrast, have hinted at covert campaigns against Iran without directly admitting involvement.

A covert war between Iran and Israel would come on top of an overt confrontation pitting Tehran against the West, involving both legal and political maneuvering and military sabre-rattling.

Washington is currently involved in an international lobbying effort to win support for new sanctions, targeting Iran's oil industry, which would bar financial institutions from the U.S. market if they do business with Iran's central bank.

Iran has threatened to respond to sanctions by shutting the Strait of Hormuz, the passageway for about one-sixth of the world's oil. Earlier this month Tehran concluded 10 days of naval exercises in the waters off of the strait, and says it plans to hold another set of sea drills in February.

In domestic politics, Ahmadinejad ousted an ally of one of his main moderate rivals, former president Akbar Hashemi Rafsanjani, from the chancellorship of the country's largest university, a state-owned newspaper reported on Thursday.

Iran daily said Ahmadinejad associate Farhad Daneshjoo received five of nine votes cast by the board of trustees of the Islamic Azad University, which enrolls more than 1.7 million students in 400 branches nationwide.

Ahmadinejad is currently under attack from both moderates backed by Rafsanjani and by clerical hardliners, and the battle often plays out in determining who controls key governmental institutions.

Supporters of Ahmadinejad had at least for two years pushed to replace current chancellor Abdollah Jasbi because of his affiliation with Rafsanjani, a former pillar of the clerical establishment, whose power base came under attacks after he lent his support to opposition leader Mir Hossein Mousavi in 2009 presidential elections.

Earlier in January a court sentenced Rafsanjani's daughter Faezeh Hashemi to six months in prison on charges of propagandizing against the ruling system.

In 2011 Rafsanjani lost his position as head of the Assembly of Experts, a clerical body which has the power to appoint the Supreme Leader of the country. He remains as the head of the Expediency Council, which is an advisory body to Khamenei, but his term will end in late February.

Contests such as the Islamic Azad university vote are seen as bellwethers of whether or not the moderates' clerical allies like Rafsanjani will remain in influential positions, or will be slowly squeezed out.