President Barack Obama said Thursday the United States is united with Gulf Arab nations in fighting the Islamic State group and sought to reassure them about US overtures to regional rival Iran. In Riyadh for talks hosted by Iran's arch-foe Saudi Arabia, Obama said Washington still had "serious concerns" about Tehran, but insisted no country had an interest in conflict with the Shiite power. Obama, on likely his final presidential visit to America's historic Gulf allies, was in Riyadh seeking to reduce tensions with the Sunni Arab states.