The United States has granted 180-day waivers on anti-Tehran sanctions to China, India, and a number of other countries in exchange for their reduction of oil purchases from Iran.
President Barack Obama's administration has now renewed waivers for all 20 of Iran's major oil buyers, after granting them to Japan and 10 European Union countries in September.
The December 7 waivers represented the second renewal for all 20 after Obama signed the sanctions into law a year ago.
The sanctions aim to choke off funding to Iran's nuclear program, which the West suspects is enriching uranium to levels that could be used in weapons.
Tehran says its nuclear program is for civilian purposes.
The United Nations' nuclear watchdog, the International Atomic Energy Agency (IAEA), has accused Tehran of covert weapons development.
The UN Security Council has approved four rounds of targeted sanctions to deter Iran from sensitive nuclear work.
Based on reporting by Reuters and AP