Story highlights President Mahmoud Ahmadinejad offers his condolences to the victims' families

"We shall persist in punishing the perpetrators of this crime," Khamenei warns

U.S. Defense Secretary Panetta: "We were not involved in any way in the assassination"

An Israeli official says he has "no idea" who is behind the attack

Iran's top cleric has blamed the CIA and Mossad, Israel's intelligence agency, for killing an Iranian nuclear scientist, Iran's state broadcaster said.

Mostafa Ahmadi Roshan, who died Wednesday, was the third Iranian nuclear scientist in two years to be killed by what Iran described as a magnetic bomb attached to his car. A fourth survived a similar assassination attempt.

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."

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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.

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.

Defense Secretary Leon Panetta reiterated that message Thursday, telling troops in Texas: "We were not involved in any way -- in any way -- with regards to the assassination that took place there.

"I'm not sure who was involved, we have some ideas as to who might be involved... but I can tell you one thing: the United States was not involved in that kind of effort, 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."

United Nations Secretary-General Ban Ki-moon strongly condemned any terrorist action or assassination of anyone, "whether scientist or civilian."

"It is not acceptable," he said via his spokesman Eduardo del Buey. "Human rights must be protected."

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.

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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.

The scientist's death comes as Western powers and Japan are tightening the screws on Tehran.

Japan announced Thursday it was prepared to gradually reduce oil imports from Iran, which supplies about 10% of its crude supply, while U.S.-led sanctions targeting Iran's central bank are driving down the value of the Iranian currency and driving up consumer prices.

At the same time, U.S. observers say a covert campaign of sabotage appears to be under way against Iran's nuclear program. In addition to the deaths of nuclear researchers, Iran's Natanz enrichment plant -- where Roshan was the deputy director for commercial affairs -- has been hit with a computer worm that Western analysts say has damaged about 10% of the centrifuges used in the enrichment process.

In the face of increased pressure, Iran has threatened to close off the Strait of Hormuz, the narrow waterway at the mouth of the oil-rich Persian Gulf.

It brought down and captured a U.S. surveillance drone over its territory and claims to have arrested a dozen American spies, including an Iranian-American former Marine who was sentenced to death this week. The United States says he was wrongly accused.