Iraqi Prime Minister Haidar al-Abadi has defended the role of Iranian-backed paramilitary force during a meeting with U.S. Secretary of State Rex Tillerson in Baghdad.

Tillerson arrived in Baghdad on Monday, hours after the Iraqi government rejected his call to dismiss the Popular Mobilization, an Iranian-backed forced that helped the government defeat ISIS and capture the Kurdish-held city of Kirkuk.

During the meeting, Abadi said Popular Mobilization “is part of the Iraqi institution,” and rejected claims that it was acting as an Iranian proxy.

“Popular Mobilization fighters should be encouraged because they will be the hope of country and the region,” he added.

Iraq and Iran have been working closely in recent years to defeat the self-proclaimed caliphate of ISIS which emerged in 2014 and captured large territories of Iraq and Syria.

Tillerson’s visit to Iraq came after a rare joint meeting with Abadi and Saudi Arabia’s King Salman in the Saudi capital of Riyadh.

The U.S. official called on Iraq to send home the Tehran-backed paramilitary units, which have helped government troops in battles against ISIS.

“Iranian militias that are in Iraq, now that the fight against Daesh and ISIS is coming to a close, those militias need to go home,” Tillerson said on Sunday in Saudi Arabia.