WASHINGTON - The United States categorically denied any role in the killing of an Iranian nuclear scientist on Wednesday and called Iranian threats to close the Strait of Hormuz "dangerous and provocative."In the fifth daylight attack on Iranian technical experts in two years, a motorcycle hitman attached a magnetic bomb to the door of 32-year-old Mostafa Ahmadi-Roshan's silver sedan as he drove down a Tehran street, killing him and a passenger.RELATED:Clinton slams Iran nuclear move, urges talksUS navy head: Iran strait 'keeps me awake at night'After the State Department initially declined to respond to Iranian charges that US or Israeli agents were responsible, the White House and US Secretary of State Hillary Clinton flatly denied any role."I want to categorically deny any United States involvement in any kind of act of violence inside Iran," Clinton told reporters during a news conference with Qatar's prime minister."We believe that there has to be an understanding between Iran, its neighbors and the international community that finds a way forward for it to end its provocative behavior, end its search for nuclear weapons, and rejoin the international community," she added.The United States suspects Iran is using its civilian nuclear program as a cover to develop atomic weapons. Iran denies this, saying its nuclear program is to generate electricity. Clinton also pushed back against Iran's threat to choke of the West's supply of oil by closing the Strait of Hormuz."The provocative rhetoric coming out of Iran in the last week has been quite concerning," Clinton said. "It has caused us and many of our partners in the region and around the world to reach out to the Iranians to impress upon them the provocative and dangerous nature of the threats to close the Straits of Hormuz."This is an international waterway. The United States and others are committed to keeping it open. It is part of the lifeline that keeps oil and gas moving around the world," she added. "It's also important to speak as clearly as we can to the Iranians about the dangers of this kind of provocation."