Iran's president has accused Israel of killing a prominent Iranian scientist long suspected by the West of masterminding a secret nuclear bomb program.
Iran's clerical and military rulers have threatened revenge for Friday's killing of Mohsen Fakhrizadeh, who Iranian media said died in hospital after assassins gunned him down in his car near Tehran.
"Once again, the evil hands of global arrogance were stained with the blood of the mercenary usurper Zionist regime," a term for Israel, President Hassan Rouhani said in a statement, according to state TV.
"The assassination of martyr Fakhrizadeh shows our enemies' despair and the depth of their hatred. His martyrdom will not slow down our achievements."
The death of Fakhrizadeh could provoke confrontation between Iran and its foes in the last weeks of Donald Trump's US presidency while complicating any effort by President-elect Joe Biden to revive the detente of Barack Obama's presidency.
Fakhrizadeh was killed in an ambush near Tehran.
The semi-official news agency Tasnim said "terrorists blew up another car" before firing on a vehicle carrying Fakhrizadeh and his bodyguards.
The military adviser to Supreme Leader Ayatollah Ali Khamenei vowed to "strike as thunder at the killers of this oppressed martyr". "In the last days of the political life of their ... ally (Trump), the Zionists seek to intensify pressure on Iran and create a full-blown war," Hossein Dehghan tweeted.
Channels of the Telegram encrypted messaging app believed to be close to Iran's elite Revolutionary Guards reported that the top security body, the Supreme National Security Council, had convened an emergency meeting with senior military commanders present.
There was silence from foreign capitals. Israel declined to comment. The White House, Pentagon, US State Department and CIA also declined to comment, as did Biden's transition team.
Fakhrizadeh has been described by Western and Israeli intelligence services for years as the mysterious leader of a covert atomic bomb program halted in 2003, which Israel and the United States accuse Tehran of trying to restore. Iran has long denied seeking to weaponise nuclear energy.
Trump, who lost his re-election bid to Biden on November 3 and leaves office on January 20, pulled the United States from a deal reached under Obama, his predecessor, that lifted sanctions on Iran in return for curbs on its nuclear program.
Biden has said he will aim to restore that agreement, although analysts say this will be a challenging goal.
Robert Malley, who served as Iran adviser to Obama and has informally advised Biden's team, said Fakhrizadeh's killing was among a series of moves that have occurred during Trump's final weeks that appear aimed at making it harder for Biden to re-engage with Iran.
In January, Trump ordered a US drone strike in Baghdad that killed Qassem Soleimani, Iran's most powerful military commander. Iran retaliated by firing missiles at a US base in Iraq, the closest the two foes have come to war in decades.
UN Secretary-General Antonio Guterres urged restraint and the "need to avoid any actions that could lead to an escalation of tensions in the region".
During the final months of Trump's presidency, Israel has been making peace with Gulf Arab states that share its hostility towards Iran.
Australian Associated Press