The leader of Iran-backed militia Hizbollah in Lebanon has issued a call to "resistance fighters" around the world to punish those responsible for the killing of top Iranian commander Qassem Soleimani.
Hassan Nasrallah, in a speech delivered on Friday morning, said he would "continue on the path" of Soleimani, who headed the external operations Quds Force for the Revolutionary Guard Corp.
"Meting out the appropriate punishment to these criminal assassins... will be the responsibility and task of all resistance fighters worldwide," he said.
Soleimani, considered to be one of the most powerful and influential figures in the Islamic Republic, was killed alongside Abu Mahdi al-Muhandis, the deputy head of the Hashed al-Shaabi paramilitary force in Iraq, in a US air strike near Baghdad airport overnight.
Tensions between the US and Iran have been rising for months, as Washington accused Tehran-backed factions of firing rockets on their troops across Iraq and on their embassy in Baghdad.
Ayatollah Ali Khamenei, Iran's supreme leader, vowed to take "severe revenge" and led an urgent meeting of the National Security Council in Tehran.
"With him gone, God willing, his work and his path will not be stopped, but severe revenge awaits the criminals who bloodied their foul hands with his blood and other martyrs' in last night's incident," he wrote on his Twitter account.
He called Soleimani the "international face of the resistance" and said he was killed by "the most cruel of those on earth". He announced three days of national mourning.
The US embassy in Baghdad urged American citizens in Iraq on Friday to "depart immediately", for fear of fallout from a US strike that the killings.
Mark Esper , US Defence Secretary, claimed the attacks had been defensive and that Soleimani had been "actively developing plans to attack American diplomats and service members in Iraq and throughout the region".
US President Donald Trump was vacationing on his estate in Palm Beach, Florida, but sent out a tweet of an American flag.
The strike followed an unprecedented war of words between Mr Trump and the Ayatollah.
Mr Trump had earlier this week ordered hundreds of US troops to the region after thousands of angry supporters of an Iranian-backed militia chanting "Death to America" besieged the mission on Tuesday, forcing staffers to take refuge in a safe room.
“They will pay a very BIG PRICE!,” he said. “This is not a Warning, it is a Threat. Happy New Year!”
Khamenei responded by saying: "You can’t do anything."
Grand Ayatollah Ali Sistani, Iraq's top Shia cleric, in an address made before Friday prayers, condemned the raid as a "wanton attack" on the country. However, he urged all parties to show restraint.
While the Trump administration celebrated the hit, others in the US warned of the potential consequences of such a high-profile assassination at tense time.
The White House traditionally gives warning to senior members of both parties in the Senate and House of Representatives ahead of major military action.
But Eliot Engel, House Foreign Affairs Committee chairman, said in a statement that the strike "went forward with no notification or consultation with Congress."
"President Trump just tossed a stick of dynamite into a tinderbox," former vice president Joe Biden said in a statement.
"Iran will surely respond. We could be on the brink of a major conflict across the Middle East."
Adel Abdel Mahdi, Iraq's caretaker prime minister, warned the US's move could start a "devastating war", one most likely to be of highest cost to Iraq.
"The assassination of an Iraqi military commander in an official post is an aggression against the country of Iraq, its state, its government and its people," Mr Abdel Mahdi said in a statement.
Russia, meanwhile, offered its condolences.
"The killing of Soleimani... was an adventurist step that will increase tensions throughout the region," the Russian foreign ministry was quoted as saying by news agencies RIA Novosti and TASS.
"Soleimani served the cause of protecting Iran's national interests with devotion. We express our sincere condolences to the Iranian people."
China on Friday appealed for restraint from all sides, "especially the United States".