The U.S. and Japan are said to have jointly intervened to prop up the Japanese yen for the first time in nearly 30 years after the currency plunged to its weakest level in decades, media reports, including initial Financial Times (FT) reporting, indicated.

The reported intervention came after the yen slid to 163.24 per dollar last month, its weakest level since 1986, as higher U.S. interest rates, rising oil prices and persistent capital outflows weighed on the currency.

The newspaper reported, citing people familiar with the matter, that the Federal Reserve Bank of New York took the unusual step of selling euros to buy yen on behalf of the U.S. Treasury on Friday.

The transactions were carried out through Goldman Sachs and Morgan Stanley, according to the Financial Times.

Washington's move came as the yen rebounded sharply last week, fuelling speculation that Japanese authorities had also intervened in currency markets.

The unit was trading at 160.53 against the dollar on Friday, having risen as high as 158 yen a day earlier.

Meanwhile, a Reuters report on Sunday said that Japan was to announce that Tokyo and Washington indeed took joint action on yen as soon as Monday.

Japanese Finance Minister Satsuki Katayama will announce on Monday that Tokyo and Washington ⁠took joint action to arrest the ⁠yen's slide to 40-year lows, two Japanese government officials told Reuters.

"Whether Tokyo was actually involved remains unclear, but the price action had all the familiar fingerprints," Stephen Innes at SPI Asset Management wrote in a comment.

Analysts cited by FT estimated Japan's intervention may have totalled about 8.45 trillion yen ($52.8 billion).

The Nikkei business daily put the amount at between 6 trillion and 7 trillion yen.

According to FT, the move would be the first coordinated U.S.-Japan effort to support the yen since 1998.

A recent Reuters photo of U.S. Treasury Secretary ​Scott Bessent's notepad during a cabinet meeting at Camp David in Maryland showed the words "To Do," ⁠followed by "Buy Japanese Yen (JPY) $5-10 bil." The Treasury did not respond when approached to comment on the matter.

While surging oil prices and concerns over debt are major reasons for the yen's weakness, a key driver is the wide gap between interest rates in Japan and those in the U.S. and other major economies.

And with markets increasingly betting that the U.S. Federal Reserve (Fed) could hike rates again before the end of the year, the divergence is more pronounced.

This gap has encouraged investors to borrow cheaply in yen and invest in other assets outside Japan with better returns, commonly known as a "carry trade," resulting in capital outflows and downside for the yen.