Iran's supreme leader promised retaliation on Saturday (November 28) for the killing of the Islamic Republic's top nuclear scientist.
Ayatollah Ali Khamenei, Iran's top authority, pledged to continue the work of Mohsen Fakhrizadeh - who Western and Israeli governments believe orchestrated a secret Iranian program to make weapons.
Fakhrizadeh's death has the potential to provoke a confrontation between Iran and its foes during the final weeks of Donald Trump's presidency.
Iran's president Hassan Rouhani was swift to blame US-ally Israel for the assassination.
He told a televised cabinet meeting that the Islamic Republic would respond, quote, "at the proper time".
Israel's N12 news channel says Israeli embassies have now been put on high alert.
But the country has declined to comment on the attack.
Fakhrizadeh was killed on Friday (November 27) when he was ambushed near Tehran - his car was sprayed with bullets.
He was rushed to hospital where he later died.
Protesters took to the streets of Tehran Saturday - burning US and Israeli flags and chanting - "No compromise, no surrender, Just war with USA!"
Tensions between the US and Iran have risen since Trump pulled Washington out of a 2015 international nuclear deal..
and ramped up harsh economic sanctions that drove down Iran's vital oil exports - crippling its economy.
Tehran, meanwhile, sped up its nuclear work.
Fakhrizadeh's death could also complicate any future efforts by President-elect Joe Biden to ease hostilities with Tehran.
The White House, Pentagon, U.S. State Department and CIA have declined to comment on the killing - as has Biden's transition team.
Last January, a U.S. drone strike in Iraq killed Qassem Soleimani, Iran's most powerful military commander.
Iran retaliated for that attack by firing missiles at a U.S. base in Iraq, the closest the two foes have come to war in decades.