The United States has denied any role in Wednesday's killing of an Iranian nuclear scientist in Tehran.

White House spokesman Tommy Vietor said the United States had “absolutely nothing to do” with the blast that killed Mostafa Ahmadi Rosha, and said the U.S. strongly condemns the attack and all acts of violence.

U.S. Secretary of State Hillary Clinton repeated the White House denial.

“I want to categorically deny any United States involvement in any kind of act of violence inside Iran.”

Iranian media said Wednesday two unidentified people on a motorcycle attached a magnetic bomb to Roshan's car, killing him and his driver and wounding a passer-by.

In statements, Iran blamed the attack on Israel and the United States. Iran's vice president, Mohammad Reza Rahimi, told state television that the incident will not stop the country from advancing its nuclear activities.

Israel too denied responsibility. But on Tuesday, Israeli military chief Lieutenant General Benny Gantaz told lawmakers that 2012 would be a “critical year” for Iran because things would happen to the country in an unnatural way.

Shahram Akbarzadeh, an Iranian analyst at the University of Melbourne in Australia, told VOA the assassination of Iranian scientists slows down the country's nuclear program because there is a limited number of qualified people in Iran.

“There aren't many people in the country who have that level of technical know-how, and it is reported that the Iranian nuclear program is not complete yet. So they are still investigating, they are experimenting, they are researching, so taking out a number of key scientists who are involved in the program would do serious damage.”

He also said Israel has made it clear it will not tolerate a nuclear-armed Iran.

Also Wednesday, Iran sent a letter to United Nations Secretary-General Ban Ki-moon, expressing deep concern over “such cruel, inhumane and criminal acts of terrorism” against Iranian scientists. The letter said Iran expects Mr. Ban and the U.N. to condemn “these inhumane terrorist acts” and to take steps toward eliminating terrorism in all its forms.

Iran's Fars agency said the 32-year-old Roshan supervised a department at the Natanz nuclear facility, Iran's main enrichment site. Iran has been enriching uranium to relatively low levels at the above-ground site.

Attackers have killed or wounded several Iranian nuclear scientists in recent years, including blasts in late 2010 that state media also attributed to bombs placed on cars by motorcyclists.