Iranian-backed Iraqi militias and their Iranian advisers must "go home" now that fight against ISIL is almost over, the US secretary of state Rex Tillerson said on Sunday.
"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," Mr Tillerson said at a news conference in Riyadh with Saudi foreign minister Adel Al Jubeir.
"The foreign fighters in Iraq need to go home and allow the Iraqi people to regain control."
Tens of thousands of Iraqis heeded a call to arms in 2014 after ISIL seized a third of the country's territory, forming the Popular Mobilisation Units (PMU) which receive funding and training from Tehran and have been declared part of the Iraqi security apparatus.
A senior US official said Mr Tillerson's call was directed at the PMU and the Quds Force, the foreign paramilitary and espionage arm of Iran's Islamic Revolutionary Guards Corps.
The US and its Gulf Arab allies are concerned by Iranian attempts to use the gains against ISIL in Iraq and in Syria to expand its influence in the region. Iran-allied militias and Iranian advisers were deployed in Syria to support the regime of Bashar Al Assad against an uprising and are now also involved in fighting ISIL there.
Mr Tillerson was in Riyadh for the launch on Sunday of the Saudi-Iraqi Co-ordination Council, which aims to promote co-operation betweeen the two countries in the fight against terrorism in the region and in the rebuilding of Iraqi areas destroyed in the battle against ISIL.
The creation of the council comes as Saudi Arabia rebuilds ties with Iraq that were severed following the 1990 invasion of Kuwait by Iraqi dictator Saddam Hussein.
Mr Al Jubeir stressed the historic ties between the two countries, which share a border, vast oil resources and many of the same tribes.
"The natural tendency of the two countries and people is to be very close to each other as they have been for centuries. It was interrupted for a number of decades. We're trying now to make up for lost ground," he said.
Iraq has come increasingly under the sway of Iran following the 2003 US invasion of Iraq that removed Saddam from power.