Soleimani, the commander of the Islamic Revolutionary Guard Corps' (IRGC) foreign operations arm, was killed by an air raid on Baghdad international airport.

"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," Pompeo told CNN.

"We know it was imminent," Pompeo said of Soleimani's plot, without going into detail about the nature of the planned operation.

"This was an intelligence-based assessment that drove our decision-making process," Pompeo added.

The top US diplomat spent the day calling officials worldwide to discuss the killing, which brought a promise of "severe vengeance" from Tehran.

Among those he phoned were Chinese politburo member Yang Jiechi, Afghanistan President Ashraf Ghani, Pakistan army chief Qamar Javed Bajwa, the foreign ministers of Britain, Germany and Russia and Abu Dhabi Crown Prince Mohammed bin Zayed Al-Nahyan.

Pompeo also spoke with Saudi Crown Prince Mohammed bin Salman, a key ally whom he thanked for Riyadh's "steadfast support and for recognizing the continuing aggressive threats posed by" the IRGC, the State Department said.

Separately, Pompeo designated as a "foreign terrorist organization" Aas'ib Ahl al-Haq (AAH), a group it says is "extensively funded and trained" by the IRGC, and also blocked its assets and those of two of its leaders.

"AAH and its leaders are violent proxies of the Islamic Republic of Iran," Pompeo said.

"Acting on behalf of their masters in Tehran, they use violence and terror to further the Iranian regime's efforts to undermine Iraqi sovereignty."