South Korean refiners and petrochemical companies are struggling to diversify imports of ultralight crude following the U.S. reimposition of sanctions on Iran over its nuclear program, industry sources said Thursday. Washington on Monday reinstated all Iranian sanctions removed under the 2016 nuclear deal but allowed South Korea and seven other countries to continue to buy Iranian oil over the next six months. South Korea is the third-largest buyer of Iranian oil. South Korean refiners and chemical firms had relied heavily on Iranian condensate for production of various petrochemical products thanks to a stable supply and price competitiveness.