The US has launched air strikes in Iraq, targeting the Iranian-backed Shia militia members believed responsible for the rocket attack that killed and wounded American and British troops at a base north of Baghdad, .
US officials said multiple strikes by US fighter jets on Thursday hit five locations and mainly targeted Kataib Hezbollah weapons facilities inside Iraq.
A Defense Department statement said the strikes targeted five weapons storage facilities "to significantly degrade their ability to conduct future attacks".
The strikes marked a rapid escalation in tensions with Tehran and its proxy groups in Iraq, just two months after Iran carried out a massive ballistic missile attack against American troops at a base in Iraq.
They came just hours after top US defence leaders threatened retaliation for the Wednesday rocket attack, making clear they knew who did it and that the attackers would be held accountable.
"The United States will not tolerate attacks against our people, our interests, or our allies," Defense Secretary Mark Esper said.
"As we have demonstrated in recent months, we will take any action necessary to protect our forces in Iraq and the region."
The Pentagon statement said the facilities hit in the precision strikes were used to store weapons used to target the US and coalition forces.
It called the counterattack "defensive, proportional and in direct response to the threat" posed by the Iranian-backed Shia militia groups.
US officials said the locations of the strikes were largely around the Baghdad region and the US expected casualties to be lower than 50.
An official with the paramilitary Popular Mobilisation Units told the Associated Press that two Iraqi federal police personnel were killed.
An Iraqi military statement said the aerial "aggression" occurred in the areas of Jurf al-Sakher, Al-Musayib, Najaf and Alexandria on the headquarters of the Popular Mobilisation Units, emergency regiments and commandos of the 9th division of the Iraqi army.
Esper told reporters at the Pentagon earlier on Thursday that President Donald Trump had given him the authority to take whatever action he deemed necessary.
On Capitol Hill earlier in the day, Marine General Frank McKenzie, the top US commander for the Middle East, told senators the deaths of US and coalition troops created a "red line" for the US but said he did not think Iran had "a good understanding of where our red line is".
Two US soldiers and one British service member were killed and 14 other personnel were wounded when 18 rockets hit the base Wednesday.
The US military said the 107mm Katyusha rockets were fired from a truck launcher that was found near the base after the attack.
US officials have not publicly said what group they believe launched the rocket attack but Kataib Hezbollah, an Iranian-backed Shia militia group, was the likely perpetrator.