US 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.