Interest rates on short-term Treasury securities rose in the government's latest auction, with yields on six-month bills hitting the highest level in more than three months. The Treasury Department sold $6.4 billion in three-month bills at an average discount rate of 5.92 percent on Monday, up from 5.78 percent last week. Another $6.4 billion was sold in six-month bills at an average discount rate of 6.28 percent, up from 6.14 percent last week. The rates on three-month bills were the highest since April 11, when they sold for 5.98 percent. The rates on six-month bills were the highest since Jan. 19, when rates averaged 6.37 percent. The new discount rates understate the actual return to investors _ 6.09 percent for three-month bills with a $10,000 bill selling for $9,850.40 and 6.58 percent for a six-month bill selling for $9,682.50. In a separate report, the Federal Reserve said the average yield for one-year Treasury bills, the most popular index for making changes in adjustable rate mortgages rose to 7.03 percent last week from 6.92 percent the previous week.