US President Donald Trump meets the press in White House. File photo

President Donald Trump's order for airstrikes on a Tehran-backed Iraqi militia group, after resisting retaliating against Iran for months, sent a clear message on Sunday that killing Americans was his red line.

But experts warned that, far from being deterred, Iran might find that line signals there is space for them to continue the kind of provocative activities that fired up tensions across the Gulf region throughout 2019.

And with Trump facing a re-election fight in 2020, some said Tehran could even step up its actions to challenge the president's promise to pull US troops out of the Middle East.

US officials said on Monday that Trump had exercised "strategic patience" during the past year in the face of Iran's stepped-up military activities in the region challenging the US and its allies.

But they said that the death Friday of a US civilian contractor in Kirkuk in a rocket attack by the Hezbollah Brigades, an Iran-supported militia, forced Trump's hand.

At least 25 members of the group were killed in retaliatory US strikes Sunday on five of their bases in Iraq and Syria.

"The president has shown a lot of restraint," Brian Hook, the State Department's Special Representative for Iran, told reporters Monday.

"We very much hoped that Iran would not miscalculate and confuse our restraint for weakness. But after so many attacks, it was important for the president to direct our armed forces to respond in a way that the Iranian regime will understand."