"This Friday afternoon, armed terrorist elements attacked a auto carrying Mohsen Fakhrizadeh, head of the Ministry of Defense's Research and Innovation Organization".
"Remember that name", Israeli Prime Minister Benjamin Netanyahu said as he identified Fakhrizedeh as the head scientist in the programme. "This cowardice-with serious indications of Israeli role-shows desperate warmongering of perpetrators", Zarif wrote.
While no one claimed responsibility for the attack, Iranian Foreign Minister Mohammad Javad Zarif pointed the finger at Israel, calling the killing an act of "state terror".
There has been no comment from Israel on the news of the assassination.
Iran's Fars news agency says the attack included "an explosion and machine gun fire".
"After a clash between the terrorists and his bodyguards, Mr Fakhrizadeh was severely injured and rushed to hospital".
"This man is the best known, the most prominent nuclear scientist in Iran".
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It was unclear if the gunmen had escaped, but Fars News reported several of the attackers had been killed in the skirmish. The president also retweeted a New York Times report on the killing, which took place as Fakhrizadeh was traveling by auto in northern Iran.
State television on its website later published a photograph of security forces blocking off the road.
Hossein Salami, chief commander of the paramilitary Guard, appeared to acknowledge the attack on Fakhrizadeh.
The military adviser to Supreme Leader Ayatollah Ali Khamenei vowed to "strike as thunder at the killers of this oppressed martyr".
The killing of Fakhrizadeh is the latest in a series of assassinations of nuclear scientists in Iran in recent years that the Islamic republic has blamed on Israel. The so-called Amad plan was shelved in 2003. Iran has long denied seeking to weaponise nuclear energy.
Dr Fakhrizadeh is thought to have headed what the United Nations nuclear watchdog and USA intelligence services believe was a coordinated nuclear weapons programme in Iran, shelved in 2003.
The scientist Mohsen Fakhrizadeh was "seriously wounded" when assailants targeted his auto before being engaged in a gunfight with his security team, Iran's defence ministry said in a statement. Tehran long has maintained its nuclear programme is peaceful. "Remember that name, Fakhrizadeh", he said, while announcing that the Israeli spy agency Mossad had stolen documents from Iran about its covert nuclear activities.
IAEA inspectors now monitor Iranian nuclear sites as part of Iran's now-unraveling nuclear deal with world powers.