Soleimani was killed at Baghdad airport ordered by President Donald Trump

Qasem Soleimani

Soleimani was killed at Baghdad airport ordered by President Donald Trump

Iran warned Friday that a "harsh retaliation is waiting" for the United States after top Iranian General Qasem Soleimani was killed by the Pentagon in an airstrike.

Iran's Supreme Leader Ayatollah Ali Khamenei vowed vengeance.

Khamenei said in a statement published by Iranian state media that the "cruelest people on earth" assassinated the "honorable" commander who "courageously fought for years against the evils and bandits of the world."

Khamenei called for three days of mourning.

"His departure to God does not end his path or his mission," the statement said, published by Fars News Agency. "But a forceful revenge awaits the criminals who have his blood and the blood of the other martyrs last night on their hands."

Soleimani was killed at Baghdad airport early Friday alongside members of Iran-backed militias operating in Iraq. The airstrike was ordered by President Donald Trump. Soleimani was killed while a convoy he was traveling in was leaving the airport. Iran funds and supports proxy militias across the Middle East.

New escalation: U.S. launched airstrike that killed Iran's Qasem Soleimani

The 62-year-old military commander played a key role in expanding Iran's influence and military reach in Iraq, Syria and Lebanon. His death marks a new, major escalation between Tehran and Washington following Trump's withdrawal from a nuclear accord between Iran and world powers. Soleimani had a hand in virtually every significant Iranian military and intelligence operation stretching back two decades, including a recent two-day siege against the U.S. Embassy in Baghdad that ended Wednesday. The assault led to the Pentagon rushing reinforcements to the diplomatic compound in Iraq.

Pro-Iran protesters want U.S. troops to leave the country. 

Soleimani was widely viewed as one of Iran's most powerful men and a heroic national figure who was standing up to Trump's "maximum pressure" Iran strategy that includes stinging economic sanctions and confrontational rhetoric and military deployments. Soleimani headed Iran's U.S.-sanctioned, elite Quds Force, part of the nation hard-line paramilitary Revolutionary Guard Corps.

Iran’s President Hassan Rouhani called Soleimani's killing a "heinous crime" and said "Iran will take revenge," while Ali Rabiei, a spokesman for Iran’s cabinet, said that Iran’s response would be severe, swift and decisive.

The Pentagon defended the targeting of Soleimani as a defensive action, saying that Soleimani had "orchestrated attacks on coalition bases in Iraq over the last several months - including the attack on December 27th - culminating in the death and wounding of additional American and Iraqi personnel."

Key figure: What we know about the strike that killed Iran's Gen. Qasem Soleimani

Sina Toossi, a researcher at the National Iranian American Council, a Washington-D.C.-based group that seeks to promote links between Americans and Iranians, noted on Twitter that Khamenei's chairing Friday of an emergency meeting of the National Security Council for the first time was a "sign of how serious & authoritative #Iran’s response to Soleimani's assassination will be."

Ayatollah Ahmad Khatami, an adviser to Iran’s supreme leader, appeared to threaten U.S. troops in the region. "This is the time to clear the region from these insidious beasts," he said while leading Friday prayers in Tehran, according to state media. The cleric added: "I am telling Americans, especially Trump, we will take a revenge that will change their daylight into a nighttime darkness."

It is unclear what Iran's response will be, but the State Department urged U.S. citizens to leave Iraq "immediately," citing "heightened tensions."

Meanwhile, officials from China to Russia criticized the Pentagon's actions.

Konstantin Kosachev, the head of the foreign affairs committee in Russia's parliament, called the U.S. airstrike "a mistake." He wrote in a social media post that "Iran may accelerate making a nuclear weapon now, even if it didn’t plan on doing it before," a reference to Tehran's insistence it is interested in nuclear activities for civilian purposes only, something Washington does not believe and is also one of the reasons Trump withdrew from the landmark 2015 nuclear accord negotiated by former President Barack Obama.

Geng Shuang, a spokesman for China's foreign ministry, said Beijing was "highly concerned" about Soleimani's killing and called for "calm and restraint."

Britain's Foreign Secretary Dominic Raab said "conflict is in none of our interests."

Israel, Iran's arch-enemy, said it was raising its military preparedness levels.

In the U.S., there was approval and condemnation for Trump's move in equal measures from Democrats and Republicans.