Riyadh - US Secretary of State Rex Tillerson on Sunday pressed the Trump administration's case for isolating and containing Iran in the Middle East and beyond.He is in Riyadh to push for Saudi Arabia and Iraq to unite in common cause to counter growing Iranian assertiveness.Tillerson called for the removal of Iranian and Iranian-supported militias from Iraq and urged other countries, particularly in Europe, to halt any business they do with Iran's Revolutionary Guards.Unproductive influencesSpeaking after participating in the inaugural meeting of the Saudi Arabia-Iraq coordination committee with Saudi King Salman and Iraqi Prime Minister Haider al-Abadi, Tillerson told reporters that an independent and prosperous Iraq would be a foil to Iran's "malign behaviour."We believe this will in some ways counter some of the unproductive influences of Iran inside of Iraq," he said at a news conference with Saudi Foreign Minister Adel al-Jubeir shortly before leaving Riyadh to fly to Qatar. Referring to Shi'ite militia in Iraq that are backed by Iran he said: "Those fighters need to go home. Any foreign fighters need to go home".Tillerson said countries outside of the region could also play a role, primarily by shunning the powerful Revolutionary Guards, which play a major role in Iran's economy and were added to a US terrorism blacklist earlier this month. Companies and countries that do business with the guards "really do so at great risk," he said.Longtime rivals"We are hoping European companies, countries and others around the world will join the US as we put in place a sanctions structure to prohibit certain activities of the Iranian Revolutionary Guards that foment instability in the region," Tillerson said.Earlier, at the coordination committee meeting, Tillerson told the Saudi king and Abadi, that the event highlighted the improving ties between the longtime rivals and showed "the great potential" for further cooperation. He noted the August reopening of a major border crossing and the resumption of direct flights between Riyadh and Baghdad.