Secretary of State Hillary Rodham Clinton on Wednesday refuted Iranian accusations that the U.S. had a role in killing a top nuclear scientist, addressing the latest escalation in tensions between Washington and Tehran in recent days. "I want to categorically deny any United States involvement in any kind of act of violence inside Iran," Clinton said. "We believe that there has to be an understanding between Iran, its neighbors and the international community that finds a way forward for [Iran] to end its provocative behavior, end its search for nuclear weapons and rejoin the international community and be a productive member of it." Tehran accused the U.S. and Israel of orchestrating the assassination on Wednesday in Tehran. An explosive was apparently attached to Mostafa Ahmadi Roshan's vehicle by an assailant riding a motorcycle, the fourth such killing in the last two years. Roshan worked at the Natanz uranium enrichment facility, one of Iran's two major sites for nuclear development, according to The New York Times. Clinton on Tuesday condemned Iran's start up of uranium enrichment at an underground facility near Qom, in northern Iran. Iran insists its goals are peaceful, but analysts say this could be another step toward weaponizing nuclear material. Also, as the U.S. is imposing tough new sanctions on Iran's Central Bank, Iranian officials have threatened to cut off access to the Strait of Hormuz, which could block most oil exports from the Persian Gulf. Top U.S. officials said military action would be taken to open it if necessary.