Japanese Finance Minister Satsuki Katayama will announce on Monday that Tokyo and Washington took joint action in the currency market to arrest the yen's slide to 40-year lows, two Japanese government officials told Reuters.Katayama is likely to stress the ‌two countries' determination ⁠to ⁠combat what they consider excessive yen declines, said the sources familiar with the ​matter, on condition of anonymity due to the sensitivity of the matter.One source, ​asked if Katayama would announce "joint action", said yes, adding, "The operation is still ongoing."The Ministry of Finance could not immediately be ​reached for comment on Sunday. U.S. Treasury ⁠officials did not ‌immediately respond to requests for comment.The expected announcement ​follows ​what market sources say were rounds of yen-buying in ⁠the market by the Japanese and U.S. authorities, the ​first such joint intervention since 2011, seeking to ​boost the Japanese currency from its lowest levels against the dollar since 1986.Japan's government conducted yen-buying, dollar-selling intervention in New York hours on Thursday, a market source told Reuters. The move came hours before the Bank of Japan's decision on Friday to ‌keep monetary policy steady while signalling a strong chance of an early interest rate hike.Treasury Secretary Scott ​Bessent, who ​said last week ⁠the yen "seems very undervalued to me", had a notepad at a Friday cabinet meeting with the words "To Do", followed by "Buy Japanese Yen (JPY) $5-10 bil", ​a Reuters photo showed.Also on Friday, the Treasury informed a number of banks that it might intervene in the yen market and that they should "stand ready for future action," a source familiar with the matter told Reuters.