The killing of Iranian General Qassem Soleimani has ignited fears the conflict between Iran and the US could spread through the whole region.
The United States announced on Friday that it is sending thousands of additional troops to the Middle East after Iran vowed "harsh retaliation" for the targeting killing of its top military leader.
A spokesperson for the US Department of Defense said that between 3,000 and 3,500 soldiers will be deployed across the region to protect US interests.
The US killed General Soleimani, the head of Iran’s Islamic Revolutionary Guard Corps (IRGC), in a targeted drone strike at Baghdad’s airport on Friday, arguing in a statement that "was actively developing plans to attack American diplomats and service members in Iraq and throughout the region".
US President Donald Trump said on Friday that the Iranian general was killed "to stop a war" and that the US is "ready and prepared" for any response. Soleimani, Trump added, was plotting "imminent and sinister attacks". "We caught him in the act and terminated him", Trump said.
The US does not want a "regime change" in Tehran, Trump said.
Following the killing, Iran's Supreme Leader Ayatollah Ali Khamenei warned that "``harsh retaliation is waiting'' and ordered three days of public mourning.
He summoned the Swiss envoy to Iran, who represents US interests in the country, twice, the first time delivering a letter to pass onto the United States.
Thousands of worshippers in the Iranian capital Tehran also took to the streets after Friday Muslim prayers to condemn the killing, waving posters of Soleimani and chanting ``Death to deceitful America.''
The attack represents a major escalation of tensions between the two countries, which have increasingly been at loggerheads since US President Donald Trump pulled out of the 2015 Iran nuclear deal.
It has ignited fears that the conflict could spread throughout the region, especially against American and Israeli interests.
World powers have called on both sides to de-escalate tension with France's deputy foreign affairs minister ``Amelie de Montchalin telling RTL radion on Friday morning that "we are waking up in a more dangerous world."
The European Union's High Representative for Foreign Affairs Josep Borrell has meanwhile called "on all actors involved and on those partners who can have an influence to exercise influence to maximum restraint and show responsibility in this crucial moment".
"The EU stands ready to continue its engagement with all sides in order to contribute to defusing tensions and reverse the dynamics of the conflict," he added.
In Iraq, which is at the centre of the current conflict between Iran and the US, Prime Minister Adel Abdul-Mahdi condemned the strike as an ``"aggression" against the country. An emergency parliamentary session is to be held on Sunday to "take decisions that put an end to the U.S. presence in Iraq."
Around 5,200 US troops are deployed in Iraq to train local forces and help in the fight against the so-called Islamic State terrorist group.
President Barham Salih has meanwhile called for "``the voice of reason and wisdom to dominate, keeping in mind Iraq's greater interests."
In the US, where Trump is currently vacationing his private club in Palm Beach, Florida, support for the strike was split along partisan lines with Republicans generally backing Trump's decision. Democrats were more nuanced, saying Soleimani deserved to be brought to justice but that the President's unilateral decision to kill him had put US citizens in the region at risk.
The Us has advised its citizens in Iraq to immediately leave the country.