Iran's Foreign Minister Javed Zarif on Friday slammed the United States (US) for the assassination of Iran Quds Force chief Qassem Soleimani and said that its "rogue adventurism" is "extremely dangerous and foolish escalation". He also called the assassination an act of "international terrorism" by the US.

"The United States' act of international terrorism, assassinating General Soleimani - who was the most effective force fighting Daesh (ISIS), Al Nusrah, and Al Qaeda - is extremely dangerous and foolish escalation. The US bears responsibility for all consequences of rogue adventurism." 

Iran's Supreme Leader Ayatollah Ali Khamenei has also expressed his severe anger over the incident and warned of "severe revenge" for "the criminals who bloodied their foul hands with his (General Soleimani's) blood", and promised that "God willing, his work and his path will not be stopped." President Hassan Rouhani also said that Iran and other "free nations of the region" would avenge "this gruesome crime".

On Friday, Iran's most senior commander Qassem Soleimani was killed by the US military at Baghdad airport in Iraq. Pentagon officials in the US later confirmed that the strike was carried out on the command of US President Donald Trump.

"At the direction of the President, the U.S. military has taken decisive defensive action to protect U.S. personnel abroad by killing Qasem Soleimani, the head of the Iranian Revolutionary Guard Corps-Quds Force, a US-designated Foreign Terrorist Organization," a statement by the United States Department of Defense read.

"General Soleimani was actively developing plans to attack American diplomats and service members in Iraq and throughout the region," it added.

Meanwhile, Trump posted a picture of the American flag on Twitter after the airstrike.

Following the incident, Iran announced three days of official mourning for the slain commander and summoned the Swiss envoy, who represents the US in Iran.

Meanwhile, the US Embassy in Iraq urged American citizens to heed the January 2020 Travel Advisory and depart Iraq immediately, in view of the heightened tensions in the region. U.S. citizens should depart via airline while possible, and failing that, to other countries via land.  Due to Iranian-backed militia attacks at the U.S. Embassy compound, all public consular operations are suspended until further notice.  U.S. citizens should not approach the Embassy," a statement by the US Embassy in Baghdad read. 

State television in Iran interrupted regular programming to let know of the General's achievements, recalling that Soleimani had on several occasions in the past foiled US plans, hinting at the fact that this assassination might have been meant as vengeful retaliation.

The Islamic Revolutionary Guard Corps of Iran has already confirmed the death of the commander of its Quds Force foreign operations arm, explicitly stating that he had been killed by US forces in Baghdad. The Guards vowed that the "resistance front" would "start a new chapter as of today".

"Certainly, the fleeting rejoicing of Americans and Zionists will turn into mourning before too long," Guards spokesman Ramezan Sharif told state television.

"We were saddened to hear the news... but our determination has increased to take revenge on the murderous America and oppressor Zionists and this will certainly happen." After the interview, Sharif broke down in tears and he and the reporter embraced.

This incident has occurred right after supporters of Popular Mobilization Forces (PMF), a militia group backed by Iran, withdrew from the United States embassy compound after two days relentless clash between the US security forces and the Iraqi militiamen on Wednesday in Baghdad as the US deployed fresh troops in the region.

On Tuesday, hundreds of militiamen and its supporters stormed the embassy compound in the green zone, smashing windows, breaking up the reception area, as a mark of protest against the US airstrike that hit Kataib Hezbollah bases over the weekend that killed 25 fighters. The US airstrike was in retaliation towards the missile attacks that killed a US contractor in northern Iraq last week.  

Before breaching the embassy, the protestors gathered outside the building, hurled water bottles and destroyed security cameras.

Blaming Iran for the attack on the US embassy, Trump tweeted, "..Iran will be held fully responsible for lives lost, or damage incurred, at any of our facilities. They will pay a very BIG PRICE! This is not a Warning, it is a Threat. Happy New Year!"

The Green Zone is a highly secured area in Baghdad which houses embassies of various countries.