Story highlights A top Iranian general accuses Israel and the United States of "state terrorism"

Massoud Jazayeri warns of a "punitive response" after a nuclear scientist dies

Top U.S. officials say categorically their country was not behind the killing

An Israeli general says he doesn't know who did it, but "doesn't shed a tear"

Israel and the United States are "supporters of state terrorism" and will suffer a "punitive response" at an "opportune time," Iran's Press TV cited a top Iranian general as saying Sunday, days after an Iranian nuclear scientist was killed.

Iran will hold "the U.S., Britain and the Zionist regime accountable for their actions," said the deputy chairman of Iran's Joint Chiefs of Staff, Brig. Gen. Massoud Jazayeri.

Jazayeri said the assassination would not hinder Iran's progress towards strategic technologies, the government-run Press TV reported Sunday, following an interview with him on Saturday.

Nuclear scientist Mostafa Ahmadi Roshan was killed Wednesday by what Iran described as a magnetic bomb attached to his car.

He was the third Iranian nuclear scientist to be killed in the past two years. A fourth survived a similar assassination attempt.

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U.S. Secretary of State Hillary Clinton "categorically" denied any role in the attacks, but urged Iran to halt its quest for a nuclear bomb.

U.S. Defense Secretary Leon Panetta Thursday told troops in Texas: "We were not involved in any way -- in any way -- with regards to the assassination that took place there.... that's not what the United States does."

Brig. Gen. Yoav Mordechai, a spokesman for the Israel Defense Forces, said on his Facebook page Wednesday: "I have no idea who targeted the Iranian scientist but I certainly don't shed a tear."

Iran's top cleric has blamed the CIA and Mossad, Israel's intelligence agency, for killing the scientist, Press TV said earlier.

Roshan's death shows that "the global arrogance spearheaded by the U.S. and Zionism has reached a deadlock in confrontation with the determined, devout and progressive nation of Islamic Iran," Supreme Leader Ayatollah Ali Khamenei was quoted as saying by the state-run Press TV.

Those responsible will not own up, Khamenei said, but the attack "has been carried out by the planning or support of CIA and Mossad [spy] services, like all other crimes of the network of international state terrorism."

Khamenei ended his message of condolence with a warning: "We shall persist in punishing the perpetrators of this crime, as well those supporting them behind the scenes."

Iran's President Mahmoud Ahmadinejad sent his condolences Friday to the families of Roshan and his driver, who also died following the attack, the state-run IRNA news agency reported. He pointed the finger at "agents of imperialism and international Zionism," a reference to Israel.

Other Iranian officials also blamed the killings on Israel and the United States, both of which have accused Tehran of pursuing a nuclear bomb -- a claim it denies.

Meanwhile, a newspaper aligned with Khamenei called Thursday for the Islamic republic to respond in kind to the killings of its nuclear scientists, suggesting Israeli officials could be targeted.

The editor of Tehran daily Kayhan, considered the organ of Khamenei's supporters, questioned why Iran should not "exercise its legal right to retaliate."

Mohammad Khazaee, Iran's ambassador to the United Nations, said the assassinations of scientists were intended to deprive Iranians of the right to peaceful nuclear energy.

"We believe that these terrorist attacks are supported by some elements -- especially within the Israeli regime as well as some quarters around the world," he said.

Iran says its nuclear program is aimed at producing civilian energy, not weapons. But it has rebuffed international demands to halt its enrichment of uranium, and the International Atomic Energy Agency says it has credible evidence that Iran has conducted weapons-related research.

The IAEA, which serves as the U.N. nuclear watchdog, reported in November that it can no longer verify that the Iranian nuclear program remains peaceful.