President Donald Trump said Friday that Iranian military commander Qasem Soleimani — who was assassinated in a US strike — should have been killed long before.

In his first substantial comments on the operation, carried out earlier on Friday at Baghdad's international airport, Trump tweeted that Soleimani “should have been taken out many years ago!” Soleimani “has killed or badly wounded thousands of Americans over an extended period of time, and was plotting to kill many more...but got caught!” Trump said.

His only earlier comments on the dramatic news included a tweet featuring a picture of a US flag and another, cryptic tweet stating that “Iran never won a war, but never lost a negotiation!“

Meanwhile, in television interviews on Friday, US Secretary of State Mike Pompeo said the US strike aimed to disrupt an “imminent attack” that would have endangered Americans in the Middle East.

Pompeo, in interviews on Fox News and CNN, declined to discuss many details of the alleged threat but said it was “an intelligence based assessment” that drove the US decision to target Qasem Soleimani, the commander of Iran's elite Quds Force.

“He was actively plotting in the region to take actions — a big action as he described it — that would have put dozens if not hundreds of American lives at risk. We know it was imminent,” Pompeo told CNN, echoing an earlier Pentagon statement on Thursday.

“These were threats that were located in the region,” Pompeo added. “Last night was the time that we needed to strike to make sure that this imminent attack ... was disrupted.”

“What was sitting before us was his travels throughout the region, his efforts to make a significant strike against Americans,” Pompeo said separately on Fox News. “There would have been many Muslims killed as well, Iraqis and people in other countries.”

In multiple statements on Twitter later in the day, Pompeo also shared that he had reached out to officials from other countries, including Pakistan's Chief of Army Staff General Qamar Javed Bajwa, German Minister for Foreign Affairs Heiko Maas, French Foreign Minister Jean-Yves Le Drian, and Afghan President Ashraf Ghani.

Meanwhile, Iran has threatened to retaliate after the overnight US air strike against the second most powerful figure in Iran that marked a dramatic escalation in the Iran-US conflict in the Middle East.

In the backdrop of rising tensions in the region, Bahrain's Gulf Air suspended flights to and from Iraq's Baghdad and Najaf cities on Friday until further notice due to security issues.

The strike was authorised by US President Donald Trump. Democratic US lawmakers, however, said they had not been briefed about any imminent Iranian plot or the planned US strike, and warned the Trump administration against pursuing war without congressional approval.

US Representative Max Rose, a US Army veteran, told CNN he wanted more information about the underlying intelligence that drove the strike, including how imminent any Iranian attack was. He also said he wanted to know what the administration's plan was to deal with an inevitable Iranian response.

Republican lawmakers echoed Pompeo, lauding the US strike and praising Trump for taking the action.

“The escalation is not on our part,” Representative Adam Kinzinger, who served in the US Air Force, told CNN.

Pompeo told Fox the strike “was aimed at disrupting that plot, disrupting further aggression and we hope, setting the conditions for de-escalation as well.” He added that the United States has fortified its assets in the region and is prepared for any possible retaliation, including a cyberattack.

“Trump and the entire United States government is prepared to respond appropriately,” he told Fox.