Iran fired 22 missiles on bases housing US troops in Iraq.
An assessment of the impact of the strikes is underway, but there were no initial reports of any US casualties.
Donald Trump - who ordered the assassination of Qassem Soleimani last week - has tweeted that "all is well."
Former US Vice President Joe Biden has hit out at Trump's decision to have Soleimani killed, as well as his round-the-clock tweeting:
