The US military launched airstrikes against a pro-Iranian group in Iraq following the deaths of two Americans and a Briton in a rocket attack, according to the military officials on Thursday.

“The operation is underway” and is targeting weapons facilities, one official said on condition of anonymity, without specifying where the strikes were taking place.

“US airstrikes targeted Hashed al-Shaabi positions in multiple provinces across Iraq, targeting logistics nodes and UAV (drone) storage units,” said a second official, adding that the Iran-backed armed faction Kataeb Hezbollah was one of those targeted.

Earlier on Thursday US Defense Secretary Mark Esper warned that “all options are on the table” to respond to the death of the three soldiers in a rocket attack on the Taji airbase.

It was the deadliest attack on an installation hosting foreign troops in several years and comes after a spate of rocket attacks targeting US troops across Iraq as well as the US embassy in Baghdad.

No group has claimed the responsibility for the attack but Washington has blamed Iran-backed factions for similar attacks.

Earlier this year, the attack at the Baghdad International Airport also killed Abu Mahdi al-Muhandis, the deputy commander of Iran-backed militias known as the Popular Mobilisation Forces (PMF).

Iran’s Supreme Leader Ayatollah Ali Khamenei swiftly pledged to take “severe revenge” for Soleimani’s assassination.

More than 5,000 US troops have been deployed in Iraq to support the Iraqi forces in the battles against Islamic State militants.

The US leads an international coalition — comprised of dozens of countries and thousands of soldiers — formed in Iraq in 2014 to confront the Islamic State, a jihadist group that Baghdad declared defeated in late 2017.

The Iraqi parliament voted to expel all foreign soldiers from the country in the wake of the killing of Soleimani, a decision that must be executed by the government.