One of the officials said five service members were seriously wounded and evacuated from the Camp Taji base and seven others were still being evaluated. Buildings on the base were in flames. The officials spoke on condition of anonymity to give details of the attack ahead of a public announcement.

Army Col. Myles Caggins, a U.S. military spokesman in Iraq, confirmed that three personnel from the U.S.-led coalition were killed and about 12 were wounded, but did not provide details about what country they were from. The U.S. military said that the names of those killed would be released after family notifications.

Caggins, in a statement, said that about 18 107mm Katyusha rockets struck the base and that Iraqi Security Forces found a rocket-rigged truck a few miles from Camp Taji. Such Russian rockets have been used in the past by Iranian-backed militia groups in Iraq.

Another U.S. official said that as many as 30 rockets were fired from the truck launcher, but 18 hit the base.

British Prime Minister Boris Johnson confirmed that a U.K. soldier had been killed in the "abhorrent" attack on the Taji base. The Ministry of Defense said the soldier, whose name has not been released, was from the Royal Army Medical Corps.

Johnson said Britain was "in close contact with our Coalition partners to establish exactly what happened."

There are about 400 U.K. soldiers in Iraq, training Iraqi and Kurdish security forces.

Officials did not say what group they believe launched the rocket attack, but Kataib Hezbollah or another Iranian-backed Shia militia group is likely.

Camp Taji, located just north of Baghdad, has been used as a training base for a number of years. There are as many as 6,000 U.S. troops in Iraq, training and advising Iraqi forces and conducting counterterror missions.

Kataib Hezbollah was responsible for a late December rocket attack on a military base in Kirkuk that killed a U.S. contractor, prompting American military strikes in response.

That in turn led to protests at the U.S. embassy in Baghdad. They were followed January 3 by a U.S. airstrike that killed Iran's most powerful military officer, Gen. Qassem Soleimani, and Abu Mahdi al-Muhandis, a leader of the Iran-backed militias in Iraq, of which Kataib Hezbollah is a member.

Kataib Hezbollah been designated a "foreign terrorist organization" by the State Department since 2009.

Later on Wednesday, Syrian opposition activists and a war monitor reported an airstrike that targeted Iranian militia positions along the Iraq-Syria border.

The Syrian Observatory for Human Rights reported 10 airstrikes carried out by three unidentified aircraft that targeted pro-Iran militias in the Boukamal region in eastern Syria, near the border with Iraq. The Britain-based Observatory which monitors the Syria war through a network of activists on the ground said at least 10 explosions were heard in the region but said there was no immediate word on casualties.

U.S. officials said the strike was not related to the Taji base strike at all. But it was not immediately clear who conducted the attack.

Syrian activist Omar Abu Layla said the unidentified airstrike targeted Iranian militia positions in the Boukamal region.

Syrian state-run media also reported an aerial attack in the Boukamal region near the Iraqi border that caused material damage.