The threat of intervention was especially pronounced Wednesday following the yen’s bold but small advance above ¥163 to the dollar overnight.

“Rate checks or actual market intervention could take place without prior warning,” cautioned Sony Bank in a report.

Japan’s currency, which has been above ¥160 to the dollar for about two months and above ¥162 to the dollar most of July, crept across the ¥163 line tentatively at first around midday in New York on Tuesday, and then weakened decisively.