South Korea has been revealed to be making rare U.S. dollar sales, with traders suspecting that South Korea and Japan are jointly intervening in the foreign exchange market.

Market sources revealed that South Korea’s foreign exchange authorities carried out a rare dollar-selling intervention on Thursday, pushing the won to a nine-month high. South Korea’s move coincided with Japan’s intervention in the New York market the same day, where Tokyo bought yen and sold dollars, pulling the currency back from a 40-year low. The won rose 2% against the dollar on Thursday, hitting 1,418 won per dollar, its strongest level since October 20 last year. The won hit a 17-year low of 1,561.50 last month, has gained more than 8% this month, and is on track for its largest monthly increase since March 2009. A foreign exchange official at South Korea’s Ministry of Finance declined to confirm the intervention. A South Korean FX trader said markets suspect coordinated intervention between Seoul and Tokyo, as the two countries had previously pledged close coordination on foreign exchange matters. On July 2, South Korea’s Vice Minister of Finance told a press conference that Seoul was maintaining close communication with Japan and other major allies on FX issues. Japan’s top foreign exchange official followed up on July 7, stating Tokyo was in close contact with Seoul’s FX officials, noting that the two countries’ financial markets sometimes exhibit similar volatility trends.