Vietnam may be named a currency manipulator, Bloomberg reported on Thursday.

Citing people familiar with the matter, Bloomberg said the number of countries on the watch list will rise to about 20 from 12 in the previous report issued in October.

Vietnam may be named a currency manipulator for artificially holding down the value of its currency, the dong, while India and South Korea are expected to be dropped from the watch list, it said.

In October, the U.S. Treasury refrained from naming China or any other trade partner as a currency manipulator as it relied on import tariffs to cut a trade deficit with China.

Bloomberg said the Treasury had so far examined the United States' 12 largest trade partners and Switzerland for the upcoming report.

"An expanded watch list could include Russia, Thailand, Indonesia, Vietnam, Ireland or Malaysia, all of which have large trade surpluses with the U.S.," it said.

Bloomberg said the number of countries on the list would increase after the U.S. Treasury Department reduced one of the three criteria it uses to determine manipulators: a current account surplus of 2 percent of gross domestic product, instead of 3 percent.