By Phil Stewart JEDDAH (Reuters) - U.S. Defense Secretary Ash Carter said after talks in Saudi Arabia that Iran's "potential for aggression" was a shared concern as he moved to bolster defense ties with Riyadh following a nuclear agreement with its arch-foe. The conservative, mainly Sunni Muslim kingdom is engaged in a contest for power with Shi'ite Iran stretching across the region. Since Salman became king in January, he has tried to build a Sunni coalition against Iran's regional allies, boosting support for rebels against Syria's President Bashar al-Assad and waging war against Yemen's Houthi militia.