The yen firmed in Asian trading Monday morning after the U.S. Treasury Department said in a report on exchange rates it would put pressure on Japan to refrain from competitive devaluation.

The Japanese currency climbed to a one-week high, having slumped to the weakest level in four years on April 11 at just short of 100 per dollar. It has depreciated 20 percent against its U.S. counterpart in the past six months, extending the drop after the Bank of Japan expanded monetary easing, which tends to debase the currency, on April 4.

The U.S. Treasury said it would pressure Japan to avoid “targeting its exchange rate for competitive purposes” in its semi-annual currency report to Congress released in Washington on Friday.

“You can’t tell me there’s not a currency war going on,” Bill Gross, who runs the world’s biggest bond fund at Pacific Investment Management Co., said in a posting on Twitter. He advised selling the yen.

The yen strengthened 0.3 percent to 98.12 per dollar as of 5:11 a.m. Sydney time and touched 97.63, the strongest since April 8. It had depreciated to 99.95 yen per dollar on Thursday, the weakest level since April 2009.

The yen advanced 0.3 percent to 128.66 per euro. The euro traded at $1.3107 from $1.3113 at the end of last week.

The Bank of Japan has doubled its monthly bond purchases to 7 trillion yen and set a two-year horizon for achieving its goal of 2 percent inflation as it seeks to revive the nation’s economy. Governor Haruhiko Kuroda said April 12 there’s no time limit to the central bank’s stimulus.

Japan’s currency plunged 22 percent in the six months to April 12, the worst performer in among 10 developed-market peers tracked by Bloomberg Correlation-Weighted Indexes. That compares with a 4.2 percent slide in the next-worst performer, the pound, and a 0.8 percent appreciation in the dollar.

The yen has lost at least 6 percent against all of its 16 major peers tracked by Bloomberg this year.

Global finance ministers, who will meet this week during the International Monetary Fund and World Bank spring meetings in Washington, signaled in February that Japan could stimulate its stagnant economy as long as policy makers refrained from publicly advocating a sliding yen.