BAGHDAD, March 12 (Xinhua) -- The Iraqi Joint Operations Command (JOC) confirmed on Thursday the killing of three international coalition members in a rocket attack on a military base housing U.S. forces.

Calling the attack a "very serious challenge and hostile act," a JOC statement said caretaker Prime Minister Mohammad Tawfiq Allawi, also commander-in-chief of the Iraqi forces, had ordered an immediate investigation to find out the perpetrators and bring them to justice.

The coalition forces are in Iraq with the approval of the Iraqi government and its mission is to train Iraqi forces and exclusively fight against Islamic State (IS) militants, the JOC statement noted.

On Wednesday evening, a previous JOC statement said 10 rockets landed on al-Taji Camp, some 20 km north of the capital Baghdad, where some U.S. troops are stationed.

No group has so far claimed responsibility for the attack, as the military bases housing U.S. troops across Iraq and the heavily fortified Green Zone in central Baghdad are frequently targeted by rebels' mortar and rocket attacks.

On Jan. 5, the Iraqi parliament passed a resolution requiring the government to end the presence of foreign forces in Iraq, just two days after a U.S. drone strike on a convoy at Baghdad airport, which killed Qassem Soleimani, former commander of the Quds Force of Iran's Islamic Revolution Guards Corps, and Abu Mahdi al-Muhandis, deputy chief of Iraq's paramilitary Hashd Shaabi forces.

More than 5,000 U.S. troops have been deployed in Iraq to support the Iraqi forces in the battles against IS militants, mainly providing training and advising to the Iraqi forces.