US President Donald Trump on Monday ordered for airstrikes on a Tehran-backed Iraqi militia group, after resisting retaliating against Iran for months, sent a clear message, killing Americans was his red line.

Experts have 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.

Earlier on Monday, US officials said that President 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 on 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 the retaliatory US strikes Sunday on five of their bases in Iraq and Syria.

Last month, the US accused Iran of “nuclear extortion” and vowed no let-up in pressure after the clerical regime, and said that it would resume uranium enrichment at the key Fordow plant.

Earlier in the month, the US government had imposed sanctions on Iran’s construction sector and the transfer of four materials related to Tehran’s nuclear or military programmes.

Earlier in September, the US imposed its first-ever sanctions against Iran’s space agency, accusing it of disguising a missile program.

In May this year, President Trump withdrew the US from the 2015 nuclear pact between Tehran and six world powers and has since reimposed and expanded punishing sanctions as part of a stated campaign of “maximum pressure” against Iran.

The Iranian president insisted that all of the steps his country had taken to reduce its commitments to the nuclear agreement were “reversible.”

Iranian officials have stressed that for any talks with the US, Washington should return to Tehran’s nuclear deal from which it withdrew in 2018 and implement its obligation under the accord.

Since October, the Hezbollah Brigades, which the Pentagon said are supported by the Iranian Revolutionary Guard Corps, carried out some 11 rocket attacks on installations in Iraq where the US and coalition forces are present.

(With inputs from agency)