Time for Shia militias over

US Secretary of State Rex Tillerson yesterday demanded that Iranian "militias" leave Iraq at a press conference in Riyadh, where the US diplomat is holding talks with top Gulf officials.

His call came after a rare joint meeting with the leaders of Iraq and Saudi Arabia.

"Certainly Iranian militias that are in Iraq, now that the fighting against (the Islamic State) is coming to a close, those militias need to go home," Tillerson said at a press conference in Riyadh. "All foreign fighters need to go home."

The United States is concerned that Iran, a Shia regional rival, will take advantage of gains against IS there and in Syria to expand the influence it gained after the US invasion in 2003, something Sunni Arab states like Saudi Arabia also oppose.

Tillerson's Gulf visit comes as part of concerted efforts to curb Shia Iran's influence in the region and follows President Donald Trump's announcement of an aggressive strategy against Tehran and his refusal to certify the Iran nuclear.

Tens of thousands of Iraqis heeded a call to arms in 2014 after IS seized a third of the country's territory, forming the Popular Mobilization Forces (PMF) which receive funding and training from Tehran and have been declared part of the Iraqi security apparatus. A senior US official said Tillerson had been referring to the PMF and the Quds Force, the foreign paramilitary and espionage arm of the powerful Islamic Revolutionary Guards Corps (IRGC).

A new joint body between Iraq and Saudi Arabia convened an inaugural meeting earlier yesterday to coordinate their fight against IS and on rebuilding Iraqi territory wrested from the group. The rare senior meeting, signalling a thaw between states that have been at loggerheads for decades, was also attended by Saudi King Salman and Iraqi Prime Minister Haider al-Abadi.

Tillerson's six-day trip will also take him to Qatar, Pakistan, India and Switzerland.