TOKYO, Aug 2 (Reuters) - 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. One source, asked if Katayama would announce "joint action", said yes, ‌adding, "The operation is still ⁠ongoing." Japan's government conducted yen-buying, dollar-selling intervention in New York hours on Thursday, a market source told Reuters. 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.