JERUSALEM — An Iranian nuclear scientist was killed in a bomb blast in Tehran on Wednesday, just days after the U.N.'s nuclear watchdog agency confirmed that Iran had begun enriching uranium in an underground facility.
The killing marked the fourth assassination of an Iranian nuclear scientist in two years and adds another incident to a series of mysterious explosions at nuclear-related sites that appear aimed to suspend Iran's drive for atomic power.
Iran immediately blamed the United States and Israel for the death of Mostafa Ahmadi Roshan, who was killed when two men on a motorcycle attached a magnetic bomb to his car, according to Iranian state media.
Mr. Roshan, 32, was a supervisor at a uranium enrichment facility in Nantaz.
"The bomb is the same as the ones previously used," said Tehran's deputy governor, Safarali Baratloo, who attributed the attack to "the Zionists" — a reference to Israel.
In previous incidents:
• Masoud Ali Mohammadi, a physics professor at Tehran Univeristy, was killed in a car bomb attack on Jan. 12, 2010.
• Majid Shahriari, a nuclear engineer for the Atomic Energy Commission, was killed in a car bomb attack on Nov. 29, 2010. Fereydoon Abbasi, a nuclear scientist at Shahid Beheshti University, was wounded in a similar attack the same day.
• Darioush Rezaeinejad, a nuclear scientist at K.N. Toosi University of Technology in Tehran, was shot dead outside his home on July 23, 2011. His wife also was killed.
Secretary of Defense Leon E. Panetta said this week that Iran is trying to achieve nuclear weapons capability but had not yet started to produce a bomb. If the Iranians start to develop a bomb, the U.S. would act, he said.
"They're going to get stopped," Mr. Panetta said.
The International Atomic Energy Agency has said it has evidence that Iran has begun enriching uranium at a new underground site at Fordow that is deep within a mountain and apparently impervious to bombing.
The enrichment is said to be up to 20 percent, far in excess of the level needed for peaceful purposes such as electricity production, as Iranian officials long have claimed.
Danny Yatom, a former head of the Mossad, Israel's intelligence agency, said Wednesday on Israeli Radio that Iran already has the know-how to produce an atomic weapon.
"I believe that, in the end, to stop the Iranians will require a military operation," Mr. Yatom said. "If the choice [for Israel] is living with an Iranian nuclear bomb — which means not to live — or a military operation, I favor a military operation."
Many analysts say that it is not yet clear whether Iran has decided to build a nuclear weapon or merely achieve the capability of doing so.
Micky Segall, a former head of the Iranian desk in Israeli military intelligence, told Israeli Army Radio that Wednesday's assassination is an example of the heightened pressure being put on Iran.
"Many bad things are happening to Iran in the recent period," he said.
In addition to the assassinations, the nuclear program has been hit by a computer virus that reportedly did extensive damage. Last month, a series of explosions hit a missile base and two facilities said to be connected to the nuclear program.
The French newspaper Le Figaro quoted Iraqi officials as saying that the Mossad was recruiting Iranian dissidents in the Kurdish region to carry out operations such as Wednesday's in Tehran.