Anti-U.S. chants at Iran nuke expert's burial TEHRAN - Thousands of mourners chanted "Death to Israel" and "Death to America" on Friday during the funeral of a slain nuclear expert whom Iranian officials accuse the two nations of killing in a bomb blast this week as part of a secret operation to stop Iran's nuclear program. The assassination of Mostafa Ahmadi Roshan has raised calls in Iran for retaliation against the U.S. and Israel, and an independent news website Friday said Iran is preparing a covert counteroffensive against the West. Roshan, a chemistry expert and a director of the Natanz uranium enrichment facility in central Iran, was killed in a brazen daylight assassination when two assailants on a motorcycle attached a magnetic bomb to his car Wednesday in Tehran. The killing bore a strong resemblance to earlier killings of scientists working on the Iranian nuclear program. State TV showed thousands of people carrying Roshan's coffin through central Tehran before it was taken to a north cemetery for burial. As it marched, the crowd chanted "death to terrorists." Iran's Supreme Leader Ayatollah Ali Khamenei, who has the final say on all state matters, called Roshan's killing a "cowardly assassination" and accused the U.S. and Israel of being behind the attack. He vowed Thursday that the perpetrators and those who ordered the attack would be punished. U.S. Secretary of State Hillary Rodham Clinton has denied any American role in the slaying and the U.S administration condemned the attack. Israeli officials, in contrast, have hinted at covert campaigns against Iran without directly admitting involvement. Watch: Clinton denies any U.S. role in killing Iranian scientist The assassination was carried out a day after Israeli military chief Lt. Gen. Benny 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." That prompted Hossein Shariatmadari, director of the hardline Iranian daily newspaper Kayhan, to ask why Iran did not avenge Roshan by striking Israel. The independent news website, irannuc.ir, quoted an unidentified security official as saying Iran is preparing a covert counteroffensive against the West in retaliation for the bomb blast. It suggested the retaliation could include assassinations abroad. "Iran's intelligence community is in a very good position to design tit-for-tat operations to retaliate for assassinations carried out by Western intelligence services," the official said, according to the website. Iran's response will be extraterritorial and extra-regional. It follows the strategy that none of those who ordered or carried out (the attacks) should feel secure in any part of the world. The website's report was also carried by the semiofficial Fars news agency, which is close to the elite Revolutionary Guard.