The United States added Japan to its "monitoring list" of major trading partners whose currency practices "merit close attention" on Thursday, while reaffirming that none had been designated as a currency manipulator.

The Treasury Department's semi-annual report looks at countries with big trade surpluses that actively intervene in foreign exchange markets to gain trade advantages.

In its report, the Treasury Department said Japan had met two out of the three criteria needed for "enhanced analysis," namely "having a material current account surplus and a significant bilateral trade surplus with the United States."

But it was not deemed to have met the third test, being "engaged in persistent one-sided intervention in the foreign exchange market."

Japan joins China, Japan, Taiwan, Malaysia, Singapore, Vietnam and Germany on the monitoring list, the US agency said in a statement, with none meeting all three criteria to merit enhanced analysis.

"No major US trading partner manipulated the rate of exchange between its currency and the US dollar for purposes of preventing effective balance of payments adjustments or gaining unfair competitive advantage in international trade during the four quarters through December 2023," the Treasury statement said.

Japan's addition comes after the authorities there began intervening in the currency markets in April to bolster the flagging yen, which has cratered from around 115 per dollar before Russia's February 2022 invasion of Ukraine to almost 160.

This was due in part to the Bank of Japan's decision to maintain ultra-low interest rates while other central banks have hiked theirs.

Since then, the authorities have spent around $62 billion to help prop up the Japanese currency, according to government data published late last month.