Finance Minister Jun Azumi's efforts to weaken the yen are being foiled, in part, by U.S. Treasurys.

Finance Ministry data released Tuesday showed Japan conducted an unannounced intervention worth ¥1.02 trillion during the first four days of November, as part of ¥14.3 trillion worth of sales in 2011. Still, the currency traded within 1.8 percent of its postwar record, clouding the outlook for he economy's export-led recovery from the Great East Japan Earthquake.

Analysts say one of the reasons for the yen's strength is that the gap between yields on two-year and five-year Treasurys is narrowing more than that for Japanese government bonds of equivalent maturity. The difference between the two spreads shrank to 0.35 percentage point Wednesday from 1.19 points a year earlier, making yields on dollar assets relatively less attractive.