Safe-haven buying has been cited as a main reason for propelling Treasuries over the past few sessions, but analysts said the move could become more entrenched if signs emerge that steep falls in stock markets may prompt policymakers to review policies.
Some also took a unanimous decision by the US central bank on Tuesday to forge ahead with its $600 billion bond-buying plan, despite a considerably more upbeat assessment of the economy and the job market, as a sign to be cautious.
"The fact that the FOMC decision was unanimous means that they are starting to take into consideration some of these uncertainties," said Akihiro Nishida, senior fixed income strategist at Mitsubishi UFJ Margan Stanley in Tokyo.
The policy statement made no mention of Japan, which is grappling with the aftermath of the country's worst earthquake on record -- and struggling desperately to avert a nuclear disaster.
Benchmark 10-year yields edged down 2 basis points to 3.19 percent as of 0352 GMT but still shy of a three-month low of 3.14 percent hit the previous day.
Two-year note yields were little changed at 0.556 percent.
Operators of a quake-crippled nuclear plant in Japan dumped water on overheating reactors on Thursday while the United States expressed growing alarm about leaking radiation and urged its citizens to stay well clear of the area.
Japanese stocks were down around 2 percent, tracking falls in the US market on concerns over the Japan crisis and after the yen surged to an all-time high against the dollar .
The Standard & Poor's 500 Index fell 1.95 percent on Wednesday, taking its weekly losses to more than 3.5 percent. Tokyo's Nikkei share average has tumbled over 11 percent this week.