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 issue.

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. United States Treasury Department officials did not immediately respond to requests for comment.

First joint yen intervention in 15 years

The expected announcement follows what market sources say were rounds of yen-buying in the market by the Japanese and US authorities, the first joint intervention since 2011, seeking to boost the Japanese currency from its lowest levels against the dollar since 1986.

The Japanese government bought yen for dollars in New York trading hours on Thursday, a market source told Reuters, with Bank of Japan data suggesting it sold as much as $58.97 billion to support the yen.

Tokyo's initial intervention came hours before the BOJ decided on Friday to keep monetary policy steady while signalling a strong chance it would raise interest rates soon. A widening rate differential with the US, where the Federal Reserve has dramatically shifted to a more hawkish stance, has been a key factor in the dollar's rise against the yen.

Read: US buys yen in a first over a decade

Shortly after BOJ Governor Kazuo Ueda held a press conference on the central bank's decision, the yen spiked in what markets suspect may have been another bout of yen-buying intervention by Tokyo.

"Going forward, as the official responsible for currency policy, I would like to respond in close coordination with monetary policy," Katayama's top currency diplomat, Atsushi Mimura, told reporters after the yen's spike on Friday, suggesting the MOF and BOJ were working hand in hand to combat the weak yen.

Also on Friday, the US 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.

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.

Concern over rising US bond yields

In another sign of bilateral coordination, the MOF made a rare post in English on X that it had "a broad range of tools to address market liquidity needs", including access to the Fed's repurchase facility providing temporary dollar liquidity.

The Fed facility, introduced in 2020 to steady markets during the COVID-19 pandemic, allows Japan to raise dollar liquidity without outright sales of US Treasuries, potentially easing funding pressures on Tokyo for intervention.

Critics have said Japan could face constraints to continued yen-buying intervention, as selling down its huge Treasury holdings to fund such action could trigger a selloff in US debt and cause an unwelcome spike in US yields.

Read More: Debt beyond control? US borrowing exceeds GDP

Some analysts saw the signs of Japan-US cooperation as driven by Washington's concern over rising Treasury yields, which could worsen if Tokyo failed to prevent a selloff in the yen and Japanese government bonds.

"Both the US and Japan face risks of inflation turning hot and leaving their central banks behind the curve," former BOJ official Nobuyasu Atago told Reuters. "They see merits in cooperating."

Highlighting Japan's concern over rising JGB yields, Economy Minister Minoru Kiuchi said on Sunday the government will step up efforts to enhance communication with markets. "It's very important to maintain market trust in Japan's fiscal sustainability," Kiuchi, known as a fan of expansionary fiscal and monetary policy, told a television talk programme.