WASHINGTON — Interest rates on short-term Treasury bills rose in Monday’s auction to the highest levels since December.

The Treasury Department auctioned $35 billion in three-month bills at a discount rate of 0.085 percent, up from 0.07 percent last week.

Another $30 billion in six-month bills was auctioned at a discount rate of 0.12 percent, up from 0.11 percent last week.

The three-month rate was the highest since these bills averaged 0.09 percent on Dec. 10.

The six-month rate was the highest since these bills averaged 0.13 percent on Dec. 26.

Separately the Federal Reserve said Monday that the average yield for one-year Treasury bills, a popular index for making changes in adjustable rate mortgages, was unchanged last week at 0.15 percent, the same as the previous week.