WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday’s auction, with rates on three-month bills rising to the highest level in three weeks while rates on six-month bills were unchanged.

The Treasury Department auctioned $28 billion in three-month bills at a discount rate of 0.075 percent, up from 0.065 percent last week. Another $32 billion in six-month bills was auctioned at a discount rate of 0.105 percent, the same as last week.

The three-month rate was the highest since those bills averaged 0.085 percent on Dec. 26.

For a $10,000 bill, the three-month price was $9,994.69, while a six-month bill sold for $9,998.10.

Separately, the Federal Reserve said Monday that the average yield for one-year Treasury bills edged down to 0.14 percent last week from 0.15 percent the previous week.