WASHINGTON (AP) - Average U.S. rates on fixed mortgages rose slightly this week, staying near three-month lows. Rates could fall next week now that lawmakers reached a deal to avert a possible government debt default. Mortgage buyer Freddie Mac says the average rate on the 30-year loan increased to 4.28 percent from 4.23 percent last week. The average on the 15-year fixed loan edged up to 3.33 percent from 3.31 percent. Mortgage rates began falling last month after the Federal Reserve held off slowing its $85-billion-a-month in bond purchases. The bond buys are intended to keep longer-term interest rates low, including mortgage rates. Longer-term rates also stayed low because of the partial government shutdown and a lack of government economic data.