Russia, China, and Italy, among others, have expressed concern while calling for de-escalation of tensions.
The US military killed General Qasem Soleimani, the powerful commander of Iran's Revolutionary Guards, in a drone strike in Iraq on Friday to protect American personnel abroad, dramatically escalating hostilities between the arch-rivals and spiking tensions in the already volatile Persian Gulf region.
Gen Soleimani, the head of Iran's elite al-Quds force and architect of its regional security apparatus, was killed when a drone fired missiles into a convoy that was leaving the Baghdad International Airport early on Friday. The strike also killed the deputy chief of Iraq's powerful Hashed al-Shaabi paramilitary force and some local Iran-backed militias.
The Pentagon confirmed the death of Soleimani in the airstrike, saying the military action was carried out at the direction of President Donald Trump "to protect US personnel abroad".
The strike comes days after Trump threatened Tehran following Iraqi supporters of pro-Iranian regime factions laying siege to the US embassy in Baghdad in the aftermath of deadly American airstrikes on a hardline Hashed faction.
The Pentagon alleged that the strike was aimed at deterring future Iranian attack plans.
"General Soleimani was actively developing plans to attack American diplomats and service members in Iraq and throughout the region. General Soleimani and his Quds Force were responsible for the deaths of hundreds of American and coalition service members and the wounding of thousands more," it said.
World Reacts
The United Nations has voiced deep concern over the escalation of tensions in the Gulf.
Deputy Spokesman for the Secretary-General Farhan Haq said, "The Secretary-General has consistently advocated for de-escalation in the Gulf. He is deeply concerned with the recent escalation."
The UN chief said this is a moment in which leaders must exercise maximum restraint. "The world cannot afford another war in the Gulf," Antonio Guterres said.
US Secretary of State Mike Pompeo said that the US was "committed to de-escalation" after reports started pouring in from across the world with countries reacting with concern over the incident.
Russia characterised the deadly US strike as "fraught with serious consequences". A Foreign Ministry statement warned that "such actions don't help resolve complicated problems in the Middle East, but instead lead to a new round of escalating tensions".
China said it was "highly concerned". "Peace in the Middle East and the Gulf region should be preserved," Chinese Foreign Ministry spokesman Geng Shuang said calling for calm and restraint from all concerned parties.
But while echoing the concerns of other Security Council members about spiralling tensions, Britain and Germany broke ranks, voicing qualified understanding for the US position.
German government spokeswoman Ulrike Demmer described the US strike as "a reaction to a whole series of military provocations for which Iran bears responsibility," pointing to attacks on tankers and a Saudi oil facility, among other events. However, she called for restraint and de-escalation.
The British foreign secretary, Dominic Raab, said, "We have always recognized the aggressive threat posed by the Iranian Quds force led by Qasem Soleimani," adding that, "Following his death, we urge all parties to de-escalate."
There were also warnings that the killing could set back efforts to stamp out remnants of the Islamic State group. A top European Union official, Charles Michel, said "The risk is a generalized flare up of violence in the whole region and the rise of obscure forces of terrorism that thrive at times of religious and nationalist tensions."
Italy also warned that increased tensions "risk being fertile terrain for terrorism and violent extremism".
Trump won the support of Israeli Prime Minister Benjamin Netanyahu "for acting swiftly, forcefully and decisively".
India said the escalated tensions in the fallout of the incident has raised alarm. Pakistan also expressed 'deep concern', urging de-escalation.
In Tehran, Iran's Supreme Leader Ayatollah Ali Khamenei said: "severe revenge awaits the criminals" behind the attack. He also announced three days of national mourning.
President Hasan Rouhani said the incident has "redoubled the determination of the nation of Iran and other free nations to stand against America's bullying and defend Islamic values".
Reaction in US
The news of Soleimani's killing generated different reactions in Washington along party lines with Republicans heaping praise on Trump and Democrats expressing concerns about the legality and consequences of the strike.
Among others, Senator Marco Rubio backed Trump's action in a tweet while Senator and US Presidential candidate Bernie Sanders called it a "dangerous escalation" that brings the US "closer to another disastrous war in the Middle East that could cost countless lives and trillions more dollars."
House Speaker and top Democrat Nancy Pelosi warned the US could not "put the lives of American service members, diplomats and others further at risk by engaging in provocative and disproportionate actions."
Ties between the US and Iran have deteriorated since Washington pulled out of the landmark nuclear deal with Tehran in 2018. The US then reimposed crippling sanctions on Iran, aiming to choke off its oil exports.
(With inputs from agencies)