BAGHDAD (AP) — Iraq's military on Thursday said it opened an investigation into a rocket attack hours earlier that killed three servicemen, including two Americans, at an Iraqi base housing coalition forces.

At least 12 coalition personnel were also injured late Wednesday by a barrage of rockets targeting Camp Taji base, located 27 kilometers (17 miles) north of Baghdad, according to a U.S.-led coalition statement. A truck rigged with 107 mm Katyusha rocket launchers was discovered by Iraqi security forces a few kilometers (miles) from the base following the attack.

A military statement from Iraq's joint operations command said caretaker Prime Minister Adel Abdul-Mahdi ordered the investigation into what he called “a very serious security challenge and hostile act."

The United Nations condemned the attack, saying it took “critical political attention away" from Iraq's ongoing domestic challenges, which threaten to create power vacuum at the seat of Iraq's government.

“The last thing Iraq needs is to serve as an arena for vendettas and external battles,” the statement said.

Heightened tensions between the United States and Iran in recent months were set in motion by a rocket attack in December on an Iraqi base that killed a U.S. contractor. American airstrikes targeting the Iran-backed militia Kataib Hezbollah followed, which led to protests at the U.S. embassy in Baghdad.

A U.S. drone strike in Baghdad then killed Iranian Gen. Qassem Soleimani, a top commander responsible for expeditionary operations across the wider Mideast. Iran struck back with a ballistic missile attack on U.S. forces in Iraq, the Islamic Republic's most direct assault on America since the 1979 seizing of the U.S. Embassy in Tehran.

Wednesday's attack coincided with what would have been Soleimani's birthday.

The potential power vacuum looming over Iraq comes after the prime minister-designate, Mohammed Allawi, withdrew his nomination earlier this month. That followed political squabbling over the naming of his cabinet. Shortly afterward, the caretaker premier, Abdul-Mahdi, said he would no longer carry out most of his official duties and called for early elections.

There are at least 5,200 U.S. troops in Iraq, training and advising Iraqi forces as part of a global coalition.