An Iranian nuclear scientist was killed by a car bomb on Wednesday that Tehran immediately blamed on Israel and Washington, worsening a tense international stand-off over its atomic programme.
The White House denied any involvement.
Iranian Vice President Mohammad Reza Rahimi told state television the murder of Mostafa Ahmadi Roshan would not stop Iran making "progress" in its nuclear activities.
Officials noted the assassination method -- two men on a motorbike attaching a magnetic bomb to the target's vehicle -- was similar to that used in the killings of three other scientists in two years.
Parliament erupted with yells of "Death to Israel" and "Death to America" after Wednesday's attack.
In Washington, National Security Council spokesman Tommy Vietor said: "The United States had absolutely nothing to do with this. We strongly condemn all acts of violence, including acts of violence like this."
The State Department said it condemned "any assassination or attack on an innocent person and we express our sympathies to the family."
Asked to comment on claims the United States and Israel were behind the attack, spokeswoman Victoria Nuland replied: "I don't have any information to share one way or another on that."
Ahmadi Roshan, 32, died immediately in the blast outside a university in east Tehran.
His driver/bodyguard later died, the Fars and ILNA news agencies reported, and a third occupant of the car was wounded.
Ahmadi Roshan was a deputy director at the Natanz uranium enrichment facility, according to the website of the university he graduated from a decade ago, Sharif University.
He was also an academic and member of the Basij militia controlled by Iran's Revolutionary Guards, a Basij statement said.
The scientist specialised in making polymeric membranes to separate gas. Iran uses a gas separation method to enrich its uranium.
Iranian media and officials accused the International Atomic Energy Agency of giving Ahmadi Roshan's name to Israeli and US intelligence. "IAEA inspectors met him recently," Mehr news agency said.
Iran's atomic energy organisation issued a statement, quoted by Arabic-language broadcaster Al-Alam, confirming Ahmadi Roshan "was working in the nuclear industry."
It said "the futile actions by the criminal Israeli regime and America will not disrupt the path the Iranian people have chosen."
The foreign ministry also blamed Israel and the United States.
Rahimi, in charge while President Mahmoud Ahmadinejad ends a Latin America tour, added: "They (Israel and the United States) should know that Iranian scientists are more determined than ever in striding towards Iran's progress."
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In Israel, a senior official said he was unaware who carried out the killing.
"I don't know who took revenge on the Iranian scientist, but I am definitely not shedding a tear," military spokesman Brigadier General Yoav Mordechai wrote on his official Facebook page.
Israeli and US media gave prominent play to the bombing.
Israeli outlets relayed comments by Israel's military chief of staff, Lieutenant General Benny Gantz, to MPs on Tuesday saying 2012 was to be "a critical year" given Iran's nuclear drive, international pressure -- "and things which happen to them (the Iranians) in an unnatural way."
Three other Iranian scientists were killed in 2010 and 2011 when their cars exploded in similar circumstances. At least two had also been working on nuclear activities.
The current head of Iran's atomic energy organisation, Fereydoun Abbasi, escaped another such attempt in November 2010.
Those attacks were viewed by officials as assassination operations carried out by Israel's Mossad intelligence service, possibly with US help.
Wednesday's killing sharpened an international confrontation over Iran's nuclear programme in which threats and counter-threats are being increasingly backed with displays of military muscle.
Western nations, the United States at the fore, are steadily ratcheting up sanctions on Iran with the aim of fracturing its oil-dependent economy.
Iran has responded by saying it could easily close the Strait of Hormuz -- a chokepoint for 20 percent of the world's oil at the entrance to the Gulf -- if it is attacked or if sanctions halt its petroleum exports.
It has also threatened to unleash the "full force" of its navy should the United States redeploy an aircraft carrier to the Gulf, where the US Fifth Fleet is based.
Washington said closing the strait is a "red line" that should not be crossed and said it would keep sending warships to the region.
Russia warned that any attack on Iran would be a "grave mistake," with deputy foreign minister Sergei Ryabkov telling the ITAR-Tass news agency its consequences "would be most far-reaching for regional and global security."
And China, which buys 20-22 percent of Iran's crude oil, said it was "not reasonable" to expect it to comply with "unilateral" US sanctions.
Tensions have also worsened following an Iranian court's death sentence this week on an American-Iranian former Marine it found guilty of spying for the CIA, and Iran's capture last month of what it said was a CIA drone.
Tehran's determination to continue its nuclear activities were underlined by the IAEA's confirmation on Monday that Iran has started enriching uranium at a second facility.
Western UN envoys on Wednesday condemned the move as a "clear breach" of UN sanctions resolutions over its nuclear programme.
Iran insists its atomic ambitions are peaceful, and has declared itself open to resuming nuclear negotiations with world powers that collapsed a year ago.