The yen has been weakening because of the gap between Japanese and US interest rates, high oil prices and concerns about Prime Minister Sanae Takaichi's spending plans further swelling Japan's enormous debts. The scale of Friday's joint operation was not known, but it was the first since 2011 when the United States and Japan -- and other G7 members -- sold yen to stop it rising after a huge earthquake. The last time Washington and Tokyo bought yen was 1998, Japan's finance minister said. The intervention came after the yen hit 163.99 per dollar last month, its weakest since 1986. "This joint action... countered excessive volatility and disorderly movements in the Japanese yen in recent months," Japan's Finance Minister Satsuki Katayama said.