The United States this week granted China and Singapore a six month waiver on sanctions against buying oil from Iran . . . Washington says they earned the reprieve because the two countries cut their oil imports from Iran . . . all of Iran’s major oil purchasers are now exempt from US sanctions . . . as part of an effort to curb Iran’s nuclear program, the US had given countries until  June 28 to demonstrate reductions in purchases from Iran or face sanctions . . . China’s reduction of oil imports from Iran was likely due to an ongoing dispute over prices and lower demand rather than concern over sanctions . . . the large number of waivers issued to US Iran sanctions suggests they will have little impact.