The average long-term U.S. mortgage rate climbed this week to its highest level in nearly 12 months, pushing up borrowing costs for prospective homebuyers at a time when rising oil prices are already squeezing household budgets. The benchmark 30-year fixed rate mortgage rate rose to 6.58% from 6.55% last week, mortgage buyer Freddie Mac said Thursday. One year ago, the average rate was 6.74%. The rate has ticked higher three weeks in a row. Higher mortgage rates can add hundreds of dollars a month in costs for borrowers, limiting homebuyers’ purchasing power. As rates rise, that can lead prospective home shoppers to delay buying a home, one reason U.S. home sales have been sluggish this year. Borrowing costs on 15-year fixed-rate mortgages, often sought by borrowers refinancing a home loan, also rose this week. That average rate increased to 5.96% from 5.93% last week. A year ago, it was at 5.87%, Freddie Mac said.