Iran's most senior nuclear scientist Mohsen Fakhrizadeh has been assassinated near the capital Tehran, the country's defence ministry has confirmed.
Fakhrizadeh died in hospital after an attack in Absard, in Damavand county.
Iran's Foreign Minister Mohammad Javad Zarif, has condemned the killing "as an act of state terror".
Western intelligence agencies view Fakhrizadeh as being behind a covert Iranian nuclear weapons programme.
"If Iran ever chose to weaponise (enrichment), Fakhrizadeh would be known as the father of the Iranian bomb," one Western diplomat told Reuters news agency in 2014.
News of the killing comes amid fresh concern about the increased amount of enriched uranium that Iran is producing. Enriched uranium is a vital component for both civil nuclear power generation and military nuclear weapons.
Iran insists its nuclear programme is exclusively for peaceful purposes.
Between 2010 and 2012, four Iranian nuclear scientists were assassinated and Iran has accused Israel of complicity in the killings.
Fakhrizadeh's name was specifically mentioned in Israeli PM Benjamin Netanyahu's presentation about Iran's nuclear programme in May 2018.