Baghdad: Major General Qassem Soleimani, the head of Iran's elite Revolutionary Guard Corps (IRGC) Quds Force and architect of its regional security apparatus, was killed in a US air raid at Baghdad's international airport at dawn on Friday.
Iraqi state television reported that aside from Soleimani, commander of Iraq's Popular Mobilization Forces (PMF) Abu Mahdi al-Muhandis was also killed in the pre-dawn raid.
The White House and the Pentagon said the attack was carried out at the direction of US President Donald Trump.
Iran's Supreme Leader Ayatollah Ali Khamenei paid tribute to Soleimani as a "martyr", and vowed a "vigorous revenge is waiting for the criminals."
Soleimani was once described by Ayatollah Khamenei, as a "living martyr of the revolution.
Iran's National Security Council convened an emergency meeting to decide Iran's response to the assassination. Reports say Ayatollah Khamenei participated in the meeting for the first time ever, denoting the gravity of the situation.
He also appointed General Esmail Qa'ani as the new head of the Revolutionary Guards' foreign operations replacing Soleimani.
"Following the martyrdom of the glorious general Haj Qasem Soleimani, I name Brigadier General Esmail Qaani as the commander of the Quds Force of the Islamic Revolutionary Guard Corps," Ayatollah Ali Khamenei said in a statement posted on his official website.
Several local news sources have reported that a number of Iranian fighter jets are patrolling the western parts of the country, which border Iraq.
A top advisor to Iranian President Hassan Rouhani said Washington "passed red lines" with its assassination of Gen. Qassem Soleimani and will "have to face its consequences".
Foreign Minister Jawad Zairf said, "The US bears responsibility for all consequences of its rogue adventurism."
Oil prices jumped more than 4 percent as the news of the death of Iranian general broke, raising concerns that escalating Middle East tensions may disrupt oil supplies.
Iran state TV cut off all broadcasts, telecasting non stop coverage to the murder of elusive general accompanied by recitation from Quran.
A three-day national mourning has been declared in Iran in honour of Soleimani.
Analysts say the deaths are a significant turning point in Iraq and the entire Middle East.
"This is a major blow on the relationship between the United States and Iraq," reported an Al-Jazeera correspondent from Baghdad. "It is a very precarious situation on which this significant development is taking place."
US Senator Chris Murphy, an opposition member of the Senate Foreign Relations Committee, warned that the incident could set off "a potential massive regional war".
Murphy responded to reports that the U.S. had killed Solemani by asking, "did America just assassinate, without any congressional authorization, the second most powerful person in Iran, knowingly setting off a potential massive regional war?"
Hillary Mann Leverett, a former White House National Security official, said the US killing of Soleimani is a "declaration of war" on Iran.
"Americans throughout the region need to be on guard. We are now in an incredibly dangerous situation. It is an incredibly dangerous course that we are on," Leverett said.
Killing Soleimani is equivalent to the Iranians "assassinating" the American defence secretary, or the commander of the US Central Command, she added.
"The president has taken this without debate in Congress, without any Congressional authority, it is probably an illegal act within the US domestic context."
Trump meanwhile posted an image of the American flag on social media following the news of Soleimani's death.
In Tehran, Soleimani's death sent shockwaves and sparked outrage, reports said.
"With the news of his assassination, there is a tremendous amount of shock and anger that could follow, not only in Iran but across the Middle East," according to correspondent reporting from Tehran.
"His name is synonymous to Iranian national pride, no matter how he has been labelled outside of the country," said Dorsa Jabbari of Al-Jazeera, adding that hymns of mourning are being played on Iranian radio and TV to mark Soleimani's death.
Soleimani was the most popular political figure in Iran, according to several local and external polls. The latest, a poll commissioned by the Center of International and Security Studies at Maryland University, found that Soleimani has increased his influence, with eight in 10 respondents saying they view him favourably.
Ali Soufan, a former FBI officer with huge experience in the Middle East, wrote in the Combating Terrorism Center's Sentinel in 2018 that "without question, Soleimani is the most powerful general in the Middle East today; he is also one of Iran's most popular living people."
"More than anyone else, Soleimani has been responsible for the creation of an arc of influence -- which Iran terms its 'Axis of Resistance' -- extending from the Gulf of Oman through Iraq, Syria, and Lebanon to the eastern shores of the Mediterranean Sea," wrote Soufan.
Ali Akbar Dareini, an expert on Iran-US affairs at the Center for Strategic Studies in Tehran believes the killing will lead to more insecurity and violence across the Middle East "exactly contrary to what the Americans claim".
"This is also a gift to Daesh and all terrorists in the region," the political analyst said.
Soleimani, along with al-Muhandis were instrumental to the two countries' fight against Daesh.
Charles Lister, a leading analyst of Iranian policy in the region says Soleimani's death "is a serious loss for Iran's regional agenda, but his 'martyrdom' will likely fuel a response that will, at least in the medium term, make up for his death. With Soleimani dead, war is coming -- that seems certain, the only questions are where, in what form and when?"
The US embassy in Baghdad meanwhile issued a statement, urging American citizens in Iraq on Friday to "depart immediately", for fear of fallout from the US strike.
"US citizens should depart via airline while possible, and failing that, to other countries via land," the embassy said in a statement.
Following a Pentagon statement that US president Donald Trump 'ordered' the attack that killed the influential Iranian general, many nations have responded given the now quickly escalating tensions between Iran and the United States. The US embassy in Baghdad urged citizens to leave Iraq immediately.
Moscow warned Friday that the US killing of top Iranian commander Qasem Soleimani in Iraq would boost tensions across the Middle East.
"The killing of Soleimani.... was an adventurist step that will increase tensions throughout the region," news agencies RIA Novosti and TASS quoted the foreign ministry as saying.
"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", after top Iranian commander Qasem Soleimani was killed in a US strike in Iraq.
"We urge the relevant sides, especially the United States, to remain calm and exercise restraint to avoid further escalating tensions," foreign ministry spokesman Geng Shuang said at a daily press briefing.
Iraq PM said US strike is a 'flagrant violation' of security agreement with US and that it 'will spark a devastating war in Iraq.
The Syrian government on Friday condemned the US killing overnight in Baghdad of top Iranian and Iraqi commanders and accused Washington of trying to fuel conflict in the Middle East.
Syria is "certain that this cowardly US aggression... will only strengthen determination to follow in the path of the resistance's martyred leaders," a foreign ministry official said.
The US killing of a top Iranian military commander has made the world "more dangerous," France's Europe minister said Friday, calling for efforts to de-escalate the deepening conflict in the Middle East.
"We have woken up to a more dangerous world," Amelie de Montchalin told RTL radio, saying President Emmanuel Macron would consult soon with "players in the region."
"In such operations, when we can see an escalation is underway, but what we want above all is stability and de-escalation," Montchalin said.