Former US Vice President and Democratic presidential candidate Joe Biden warned that “Iran will surely respond.”

“President Trump just tossed a stick of dynamite into a tinderbox, and he owes the American people an explanation of the strategy and plan to keep safe our troops and embassy personnel, our people and our interest, both here at home and abroad, and our partners throughout the region and beyond,” he said on Twitter.

US Democratic leaders in both chambers of Congress issued statements condemning Trump's ordered strike, saying the president did not obtain congressional approval.

"We cannot put the lives of American service members, diplomats and others further at risk by engaging in provocative and disproportionate actions," said US House of Representatives Speaker Nancy Pelosi.

"Tonight's airstrike risks provoking further dangerous escalation of violence. America – and the world – cannot afford to have tensions escalate to the point of no return," Pelosi said in a statement.

"The Administration has conducted tonight's strikes in Iraq targeting high-level Iranian military officials and killing Iranian Quds Force Commander Qasem Soleimani without an Authorization for Use of Military Force (AUMF) against Iran. Further, this action was taken without the consultation of the Congress," she added.

US Representative Ilhan Omar attacked the president Thursday night for his order to kill the top Iranian general.

“So what if Trump wants war, knows this leads to war and needs the distraction? Real question is, will those with congressional authority step in and stop him? I know I will,” she tweeted.

US Senator and Democratic presidential candidate Elizabeth Warren called the US attack a “reckless move” that “increases the likelihood of more deaths and new Middle East conflict.”

Senator Tom Udall said the strike could put “US forces and citizens in danger and very possibly sinking us into another disastrous war in the Middle East that the American people are not asking for and do not support.”

“I urge members of Congress on both sides of the aisle to show courage on this issue, and I urge the Trump administration to change course and pursue diplomacy before we are entangled in yet another war in the Middle East with no end in sight,” he added in a statement.

The UN special rapporteur on extrajudicial executions, Agnes Callamard, also condemned the killing of Soleimani.

“The targeted killings of Qasem Soleiman and Abu Mahdi Al-Muhandis are most likely unlawful and violate international human rights law: Outside the context of active hostilities, the use of drones or other means for targeted killing is almost never likely to be legal,” she wrote on Twitter.

Reporter’s code: 50101