Japanese Finance Minister Taro Aso said that one-sided speculative movements in the dollar-yen rate are “extremely concerning,” the Nikkei newspaper reported Sunday.

Japan will carefully watch currency markets to ensure speculation doesn’t continue and will take action as may be necessary, Aso told reporters at Haneda Airport late Saturday.

The yen surged against the dollar to 106.5 on Friday for an 18-month high after the Bank of Japan didn’t deliver further monetary easing as markets had expected. The yen has climbed about 12 percent this year against the dollar.

Aso’s comments came as the U.S. is stepping up rhetoric on foreign-exchange practices with a semi-annual Treasury report saying Japan is among countries that have been put on a watch list.

Aso said the April 29 report in no way constrains Japan’s ability to respond should any action be needed, according to the Nikkei report.

The U.S. also put China, Germany, South Korea and Taiwan on the list, saying it will monitor policies to gauge whether they provide an unfair trade advantage over the U.S. The five countries have met two of three criteria used to judge practices. If a country meets all three criteria, it could eventually be cut off from some U.S. development financing and excluded from U.S. government contracts.

China, Japan, Germany and South Korea were flagged as a result of their trade and current-account surpluses, the Treasury Department said. Taiwan made the list because of its current-account surplus and persistent intervention to weaken the currency, according to the Treasury.

The report noted that Japan has not intervened in the foreign exchange market in more than four years.

The Treasury said it’s increasingly important that Japan use all policy levers, including fiscal policy and structural reforms, to lift growth. Last month, Treasury Secretary Jacob J. Lew urged Japan to focus on boosting domestic demand instead of exports as the yen rises. (Bloomberg)