An Iranian military commander said Iran would strike back against killers who shot dead a suspected top nuclear scientist.
Tehran is blaming Israel for the assassination of researcher Mohsen Fakhrizadeh after he was shot dead near the capital on Friday.
Officials labelled it a 'terrorist' attack and are accusing Israel of a role in his death.
“We will strike as thunder at the killers of this oppressed martyr and will make them regret their action,” Hossein Dehghan, military adviser to Iranian Supreme Leader Ayatollah Ali Khamenei, wrote in a tweet.
The United Nations Secretary-General Antonio Guterres urged restraint.
"We have noted the reports that an Iranian nuclear scientist has been assassinated near Tehran today.
"We urge restraint and the need to avoid any actions that could lead to an escalation of tensions in the region," Guterres' spokesman Farhan Haq said.
Reports claim Western officials have long believed Fakhrizadeh was the head of Iran's secretive nuclear programme.
The Ministry of Defence confirmed the death of the physics professor and leading figure in Iran's Revolutionary Guard.
Iran's Foreign Minister Mohammad Javad Zarif claimed there were indications of an Israeli role in the scientist's death.
He demanded that Western nations, and particularly the European Union, condemn the 'act of state terror' immediately.
He wrote in a tweet: "Terrorists murdered an eminent Iranian scientist today.
"This cowardice—with serious indications of Israeli role—shows desperate warmongering of perpetrators Iran calls on int'l community—and especially EU—to end their shameful double standards & condemn this act of state terror."
The Pentagon has declined to comment on the incident, Reuters reports.
Western officials and experts believe Fakhrizadeh played a pivotal role in suspected Iranian work on its nuclear programme in the past.
It is believed he was a key figure in developing the means to assemble a nuclear warhead behind the facade of a declared civilian uranium enrichment programme.
Iran denies ever having sought to develop a nuclear weapon.
A landmark report by the UN nuclear watchdog in 2011 identified Fakhrizadeh as a central figure in suspected Iranian work to develop technology and skills needed for atomic bombs,and suggested he may still have a role in such activity.
Believed to be a senior officer in the elite Revolutionary Guard, Fakhrizadeh was the only Iranian the report identified.
The UN. International Atomic Energy Agency has long wanted to meet Fakhrizadeh as part of a protracted investigation into whether Iran carried out illicit nuclear weapons research.
Showing no sign it would heed the request, Iran acknowledged Fakhrizadeh's existence several years ago but said he was an army officer not involved in the nuclear programme, a diplomatic source with knowledge of the matter told Reuters.
He was also named in a 2007 UN resolution on Iran as aperson involved in nuclear or ballistic missile activities.
