The decision, announced Thursday -- the day the stringent sanctions went into effect -- spares all 20 of Iran's major oil buyers from the sanctions intended to reduce Iran's oil exports.
"A total of 20 world economies have now qualified for such an exception," Secretary of State Hillary Clinton said in a statement. "Their cumulative actions are a clear demonstration to Iran's government that Iran's continued violation of its international nuclear obligations carries an enormous economic cost."
China is the world's No. 1 buyer of Iranian oil. Singapore is the world's third-largest oil refining center.
The two countries received renewable 180-day exemptions under a sanctions law President Barack Obama signed in December because they "have each significantly reduced the volume of crude oil purchases from Iran," a senior administration official told reporters Thursday afternoon.
The export reduction is intended to pressure Iran to suspend its enrichment of uranium to 20 percent and comply with other Western demands.
Clinton's exemption announcement came five days before a fourth round of negotiations -- this time in Istanbul, Turkey -- between six world powers and Iran over its nuclear program.
The administration determined March 20 that Japan and 10 European Union nations -- Belgium, Britain, the Czech Republic, France, Germany, Greece, Italy, the Netherlands, Poland and Spain -- qualified for the exemptions.
India, Malaysia, South Africa, South Korea, Sri Lanka, Taiwan and Turkey won exemptions June 11.
Financial institutions in all 20 countries faced being cut off from the U.S. banking system starting Thursday if the countries did not reduce their imports enough to be given a six-month exemption from the sanctions.