US President Donald Trump has ordered new retaliatory economic sanctions on Iran as his administration faces questions over its drone strike on an Iranian general that ignited the latest crisis with the Islamic Republic.

Mr Trump issued an executive order adding additional US sanctions to an already long list his administration had imposed.

The aim is to force Iran to accept a new agreement that would curb its nuclear program and halt support for militant groups throughout the Middle East.

The president, in a statement announcing the new measures, referenced Iran’s nuclear program and use of proxy forces throughout the region while noting that the Iranians have threatened US service members, diplomats and civilians – an apparent reference to his administration’s justification for killing General Qassem Soleimani in a drone strike last week in Baghdad.

“The United States will continue to counter the Iranian regime’s destructive and destabilising behavior,” he said.

.@SecPompeo: Today President @realDonaldTrump is delivering on the pledge he made the day after #Iran attacked American forces in Iraq. We are levying new sanctions on the Islamic Republic of Iran. pic.twitter.com/9k97VMNL7e

— Department of State (@StateDept) January 10, 2020

But Mr Trump and his administration faced continuing questions over its claims of an “imminent” threat as justification for killing General Soleimani, setting off a furore that may not yet be contained.

Secretary of State Mike Pompeo and others in the administration have said the threat posed by General Soleimani was imminent, but they have not offered evidence to support that.

Members of Congress said officials did not provide sufficient detail or justification in briefings this week.

Define what you mean by imminent, Mr Pompeo was asked at Friday’s White House session.

“I don’t know exactly which minute,” Mr  Pompeo said. “We don’t know exactly which day it would have been executed, but it was very clear. Qassem Soleimani himself was plotting a broad, large-scale attack against American interests and those attacks were imminent.”

Last night Pompeo told Fox News, "…We don’t know precisely when and we don’t know precisely where, but it was real." Today, a pause before answering with this: https://t.co/Lmw6dhgYjS

— Kaitlan Collins (@kaitlancollins) January 10, 2020

Both Mr Pompeo and Mr Trump said US embassies were threatened.

The Secretary of State broadened it to include “American facilities,” including military bases throughout the region. “This was going to happen, and American lives were at risk,” he said.

He spoke amid revelations by US officials that the American military had tried, but failed, to kill another senior Iranian commander on the same day that General Soleimani was killed.

It was apparently part of an effort to cripple the leadership of Iran’s Quds force, which the US has designated a terror organisation along with the larger Islamic Revolutionary Guard Force.

House Speaker Nancy Pelosi called the killing of General Soleimani “provocative and disproportionate” and other members said they were unconvinced after a closed-door briefing on the intelligence.

“President Trump recklessly assassinated Qassem Soleimani,” said Representative Pramila Jayapal, a Washington State Democrat.

“He had no evidence of an imminent threat or attack.”

Smoke billows from a sentry box at an entrance of the US embassy in the capital Baghdad. Photo: Getty

The new sanctions were in immediate response to Iran’s firing of a barrage of missiles at American bases in neighbouring Iraq this week after to the killing of General Soleimani. No one was injured.

In 2018, Mr Trump withdrew from the nuclear agreement signed under President Barack Obama that traded curbs on the program for the easing of sanctions.

Since then, the administration has added additional economic measures that have created hardship in Iran and brought its oil revenue to historic lows but have failed to bring the Iranian government to the negotiating table.

The sanctions added on Friday include measures aimed at eight senior Iranian officials involved in what Treasury Secretary Steven Mnuchin called “destabilising” activities throughout the Middle East as well as Tuesday’s missile barrage.

Those measures, which would freeze any assets the officials have in US jurisdiction and prohibit financial transactions with them, are largely symbolic since such senior figures are unlikely to have assets under American control.

But other measures could have a significant effect on strategically important sectors of the Iranian economy.

The executive order grants the administration power to place anyone involved, even indirectly, in the construction, manufacturing, textile or mining sector on a global financial blacklist.

It also targets 17 of the largest steel and iron manufacturers – one of the few growth spots in the hobbled Iranian economy.

-AAP