As Iran blamed Israel for the killing of a top Iranian nuclear scientist on Friday, a former CIA chief called the assassination of Mohsen Fakhrizadeh a "highly reckless" and "criminal act".
Fakhrizadeh headed the ministry of defence's research and innovation organisation. He was severely injured in an attack on Friday in the town of Absard in the Tehran region and died later from his wounds in a hospital.
Iran President Hassan Rouhani and Foreign Minister Mohammad Javad Zarif blamed Israel for the killing.
In a series of tweets on Saturday, John Brennan, who headed the Central Intelligence Agency (CIA) as its Director during second term of President Barack Obama from 2013-2017, said, "This was a criminal act & highly reckless. It risks lethal retaliation & a new round of regional conflict. Iranian leaders would be wise to wait for the return of responsible American leadership on the global stage & to resist the urge to respond against perceived culprits."
"I do not know whether a foreign government authorized or carried out the the murder of Fakhrizadeh. Such an act of state-sponsored terrorism would be a flagrant violation of international law & encourage more governments to carry out lethal attacks against foreign officials," he said in a nother tweet.
"These assassinations are far different than strikes against terrorist leaders & operatives of groups like al-Qaida & Islamic State, which are not sovereign states. As illegitimate combatants under international law, they can be targeted in order to stop deadly terrorist attacks," Brennan added.
Fakhrizadeh was Iran's most renowned nuclear scientist and his killing threatens to escalate tensions over Iran`s nuclear programme with the US and its close ally Israel.
President Rouhani said on Saturday his country would respond "in due course" but that Fakhrizadeh`s killing would not push Iran into making hasty decisions.
"Iran's enemies should know that the people of Iran and officials are braver than to leave this criminal act unanswered," he said in a televised cabinet meeting.
"In due time, they will answer for this crime," he added.
In an earlier statement, the president accused the "the mercenaries of the oppressive Zionist regime" - referring to Israel - of being behind the attack.
"The assassination of martyr Fakhrizadeh shows our enemies' despair and the depth of their hatred... His martyrdom will not slow down our achievements."
Iran Supreme Leader Ayatollah Ali Khamenei also called for the "punishing" of the perpetrators of the attack "and those who commanded it", in a tweet on Saturday.
His military adviser, Hossein Dehghan, had earlier vowed to "strike" the attackers like thunder.