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 with a series of tit-for-tat confrontations.

"To the leaders of Iran - DO NOT KILL YOUR PROTESTERS," Trump tweeted, warning 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 stilling 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."