"Those militias need to go home," Tillerson said. "Any foreign fighters in Iraq need to go home and allow the Iraqi people to regain control of areas that had been overtaken by ISIS and Daesh that have now been liberated. Allow the Iraqi people to rebuild their lives with the help of their neighbors."
Tillerson was speaking in Riyadh alongside the Saudi Arabian foreign minister days after US-backed forces declared the liberation
of Raqqa, ISIS's de facto capital in Syria. US forces remain in Iraq, supporting the government there and combating ISIS, as do forces backed by Iran.
The US under President Donald Trump has assumed a more aggressive posture towards Tehran, with Trump recently threatening to exit a multilateral agreement with Iran over its nuclear program.
Tillerson referenced this posture during his remarks in Saudi Arabia and said he spoke about it with Saudi leadership. He said both the US and Saudi Arabia are warning countries from doing business with the Iranian Revolutionary Guard Corps, an Iranian military body the US has accused of human rights abuses and sowing instability throughout the region.
"Both of our countries believe that those who conduct business with the Iranian Revolutionary Guard, any of their entities -- European companies or other companies around the globe -- really do so at great risk," Tillerson said.
It's not clear how many Iranians are fighting in Iraq. In May this year, a senior commander of Iran's Islamic Revolutionary Guard Corps
was reportedly killed fighting ISIS west of the city of Mosul. The large majority of those who make up the militia forces in Iraq, those who operate under the banner of the Popular Mobilization Units (PMU), are Iraqi Shiites, and those too are backed, equipped and trained by Iran. Those number at least 100,000
, according to modest estimates.
At the same time, at least 1,500 Saudis had traveled to Iraq and Syria to join ISIS
over the years, and Baghdad had long accused Saudi Arabia of turning a blind eye to Sunni militants crossing its territory to enter Iraq to take part in the country's sectarian conflict.
In 2007, the US military reported that around 40% of all foreign militants
targeting US troops and Iraqi civilians and security forces had come from Saudi Arabia. Half of the Saudi fighters who arrived in Iraq during that time went there to be suicide bombers, the US military said.
After his stop in Saudi Arabia, Tillerson shuttled to Qatar, which is a party to an ongoing diplomatic crisis involving several Gulf nations.
Saudi Arabia, the United Arab Emirates, Egypt and Bahrain accuse Qatar of financing terrorism and have cut off diplomatic ties to that country and initiated an economic boycott of it.
Tillerson, who has worked to broker an end to the dispute for months, said he and Trump are still looking to resolve the tensions, but said when he spoke most recently with Saudi Crown Prince Mohammad bin Salman, he did not believe the Saudis were ready to come to the table.
"I asked him to please engage, please engage in dialog," Tillerson said, speaking from Qatar's capital city of Doha alongside that nation's foreign minister. "There is not a strong indication that parties are ready to talk yet, and so we cannot force talks on people who are not ready to talk."
Tillerson's third visit to Riyadh as Secretary of State was part of the inaugural meeting of a council established between Saudi Arabia and Iraq.
The two Arab nations have only recently re-established many bilateral ties. While Iran re-opened its embassy in Baghdad shortly after the invasion of Iraq in 2003, Saudi Arabia re-opened its own in 2015. The border between Iraq and Saudi Arabia, closed after Iraq invaded Kuwait in 1990. It only re-opened for trade in August this year