Asian stocks struggled as the new month began after a turbulent July marked by wild swings as investors fretted over massive capital spending on AI and whether it will provide returns quickly enough.
Japan's Nikkei closed 1% lower, while South Korea's KOSPI slid more than 5%. This weighed on the MSCI's broadest index of world shares, leaving it flat.
The Japanese yen strengthened over 0.5% to 156.77 per U.S. dollar after a jump earlier in the day to its strongest since early May at 155.2, putting traders on alert for another bout of intervention.
Japan and the U.S. conducted coordinated yen-buying and will not hesitate to take further action, Japan's Finance Ministry said on Monday, confirming a rare bilateral action to halt the yen's slide to 40-year lows.
U.S. Treasury Secretary Scott Bessent also said the United States would consider increasing in coming months the size of the Federal Reserve's repurchase facility providing temporary dollar liquidity, calling the tool an "important backstop".
"Bessent's comments arguably carry more weight than the intervention itself," said Matt Simpson, senior market analyst at StoneX. "It feels like a safe bet that the Japanese yen has troughed for the year. The words 'joint intervention' carry a lot of weight in these markets and is a term rarely used."
Trump said on Sunday the United States was helping Japan prop up the yen as a sign of friendship and to help the world economy.
Tokyo's solo intervention conducted between late April and early May caused only a brief yen rebound, while a rate hike in June by the Bank of Japan provided little support, underscoring the challenge policymakers face of rising oil prices and a wide interest-rate differential against other major economies.
The yen had been rooted near 40-year lows around 164 per U.S. dollar in recent weeks before the latest interventions, with net short yen positions of roughly $12.5 billion, the highest in two years, data from a U.S. regulator showed.
Bessent repeated calls for further interest-rate hikes by the Bank of Japan.
Elsewhere, U.S. Treasury yields retreated as oil prices fell.
The yield on the 30-year bond fell over 5 basis points to around 5.22%, easing away from the 19-year high it touched last week.
It had soared around 37 basis points in July as investors grappled with confusion around the Iran war and the policy outlook for the Federal Reserve.
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