US President Donald Trump warned Iran Sunday against killing protesters who have risen up over the regime's downing of a civilian airliner as his defense secretary left the door open to talks with Tehran without preconditions.

Trump's salvo came as Iran's Islamic regime faced a challenge from angry street protests, having come to the brink of war with the US after a series of tit-for-tat confrontations.

"To the leaders of Iran - DO NOT KILL YOUR PROTESTERS," Trump tweeted, warning that the world and "more importantly, the USA is watching."

In an interview with CBS's "Face the Nation" just before the tweet, US Defense Secretary Mark Esper said Trump was still willing to hold talks with Iran's leaders.

"We're willing to sit down and discuss without precondition a new way forward, a series of steps by which Iran becomes a more normal country," Defense Secretary Mark Esper said on CBS's "Face the Nation."

And if something happened to the protesters? Esper replied: "The president has drawn no preconditions other than to say we're willing to meet with the Iranian government."

Long-standing US-Iran tensions have soared since January 3 when missiles fired from a US drone killed a top Iranian commander, Qasem Soleimani, near Baghdad's airport.

Iran responded with a barrage of missiles at two US bases in Iraq, inflicting no casualties in what was seen as an attempt to prevent a spiral of escalation.

But hours later, an Iranian Revolutionary Guard unit shot down a Ukrainian passenger jet shortly after takeoff from Tehran.

The death of all 176 people aboard and Iranian leader's belated admission its forces mistook the plane for a cruise missile has set off angry protests against the regime.

The British ambassador to Iran was briefly arrested Saturday after attending a memorial service to the victims at Tehran's Amir Kabir University.

"I just think you see a very corrupt regime that the Iranian people are finally standing up and trying to hold them accountable," Esper said.

On another Sunday talk show, national security advisor Robert O'Brien said the Iranian regime was "reeling from maximum pressure."

"They are reeling from their incompetence in this situation. And the people of Iran are just fed up with it," he said on ABC's This Week.

Meanwhile, said Esper, the US believes it has disrupted the plots that it says precipitated Soleimani's killing, and expects no further Iranian retaliation.

"We do not expect any further attacks. But if you look at what's happening on the ground today, you have just yesterday in Tehran and other cities, Iranians chanting, 'death to the Ayatollah'." he said.

Esper and O'Brien defended the intelligence that led the administration to claim Soleimani was plotting imminent attacks on US troops and diplomats in the region.

But other than in the case of the US embassy in Baghdad, Esper would not confirm Trump's claim in a recent Fox News television interview that four US embassies in the region were among Soleimani's target.

Asked if there was specific evidence in the intelligence to support the claim, Esper said, "I didn't see one with regard to four embassies."

The administration has come under withering fire from Democrats -- and at least two Republican senators -- for refusing to share the intelligence with members of Congress.

Senior congressional leaders -- the so-called Gang of Eight -- were briefed January 8 as skepticism mounted about the administration's rationale for a killing that raised the risk of war with Iran.

"We had exquisite intelligence and the intelligence showed that they were looking at US facilities throughout the region and that they wanted to inflict casualties on American soldiers, sailors, airmen, marines, as well as diplomats," O'Brien said on NBC's Meet the Press. "The threat was imminent. I saw the intelligence."

Adam Schiff, chair of the House Intelligence Committee, said there was no discussion in the briefing given to the Gang of Eight about plots against four embassies.

"In the view of the briefers, there was plotting, there was an effort to escalate, a big plan, but they didn't have specificity," he said.

"So when you hear the president out there on Fox, he is fudging intelligence." he said on Face the Nation.

House Speaker Nancy Pelosi said the congressional leadership should have been informed of the strike against Soleimani in advance.

"I don't think the administration has been straight with the Congress of the United States." she said on Meet the Press.