Discounts on Iranian oil sold to China have widened to their largest margins in more than a year, as tightening sanctions on both Russia and Iran curb purchases by independent Chinese refiners already constrained by limited crude import quotas, trade sources said on Wednesday. The sanctions have also prompted some buyers in China and India—the two biggest importers of Russian oil—to temporarily halt purchases, driving a sharp drop in Russian crude prices and adding unsold Russian cargoes to an already ample supply of Iranian oil, the sources added.