The average long-term US mortgage rate has surged to its highest point in nearly a year this week, significantly increasing borrowing costs for prospective homebuyers across the nation.According to mortgage buyer Freddie Mac, the benchmark 30-year fixed-rate mortgage climbed to 6.55 per cent, up from 6.49 per cent just last week. This rise comes despite the average rate standing at 6.75 per cent a year ago.Such an increase in mortgage rates can add hundreds of dollars to monthly payments for borrowers, further constraining the purchasing power of homebuyers at a time when affordability challenges continue to deter many aspiring homeowners from entering the market.Mortgage rates are influenced by a confluence of factors, ranging from the Federal Reserve’s interest rate policy decisions to the expectations of bond market investors regarding the economy and inflation. These rates typically mirror the trajectory of the 10-year Treasury yield, which lenders utilize as a benchmark for pricing home loans.(AFP/Getty)Rates have been on an upward trend this year, largely due to the war with Iran, which has sharply driven up crude oil prices and fueled expectations of hotter inflation. This has, in turn, pushed long-term bond yields higher than they were before the conflict began in late February, causing mortgage rates to follow suit. The 10-year Treasury yield reached 4.57 per cent at midday Thursday, up from 4.54 per cent a week prior and 3.97 per cent in late February.The current average rate for a 30-year mortgage is now the highest since August 28, when it registered 6.56 per cent. As recently as late February, the average rate briefly dipped below 6 per cent for the first time since late 2022.Borrowing costs for 15-year fixed-rate mortgages, often favored by those refinancing existing home loans, also saw an increase this week. That average rate rose to 5.93 per cent from 5.82 per cent last week, Freddie Mac reported, compared to 5.92 per cent a year ago.While average long-term mortgage rates remain below last year's levels, their recent upward trajectory has exerted pressure on home sales throughout this year.