Two officials said an announcement of the six-month extensions was expected from the State Department on Friday. The officials spoke on condition of anonymity because they were not authorized to publicly preview the step.
In addition to China, India and South Korea, the waivers will apply to Malaysia, Singapore, South Africa, Sri Lanka, Turkey and Taiwan. All nine were originally granted six-month renewable exemptions from the sanctions in June.
The exemption means that banks and other financial institutions based in those places will not be hit with penalties under U.S. law enacted as a way of pressuring Iran to come clean about its nuclear program.
A total of 20 countries and Taiwan have been granted the waivers. The others—Belgium, Britain, the Czech Republic, France, Germany, Greece, Italy, the Netherlands, Poland, Spain and Japan—will come up for review in March.
The administration says the exemptions are a sign that pressure on Iran is increasing.
In June, U.S. officials said Iran's oil exports have declined from about 2.5 million barrels a day last year to between 1.2 million and 1.8 million barrels a day, choking a key source of revenue for the regime, which remains defiant over international demands that it prove that its nuclear program is peaceful.
The U.S. sanctions target foreign financial institutions that do business with Iran's central bank by barring them from opening or maintaining correspondent operations in the United States. The sanctions would apply to foreign central banks only for transactions that involve the sale or purchase of petroleum or petroleum products and then only if the Obama administration determined that there was enough non-Iranian oil available to make up the difference without disrupting oil markets.