The 30-year, fixed-rate mortgage edged up to 4.55% this week as analysts noted a slight uptick in mortgage rates over worries that a U.S. debt default could send loan rates soaring. "Industry analysts have made it clear that if the United States defaults and the national debt is downgraded, mortgage rates could spike immediately," Bankrate said in its mortgage rate report. "But the uncertainty over what Congress will decide over the next few days has already started to shake the mortgage world, as investors question if it's still safe to invest in U.S. bonds." The 30-year FRM rose to 4.55% from 4.52% last week and 4.54% a year ago, according to Freddie Mac's Primary Mortgage Market Survey report. The same report said the 15-year FRM remained virtually unchanged from last week, hovering at 3.66%. A year ago, the 15-year, FRM hit 4%. Freddie's rate report showed the 5-year, Treasury-indexed, hybrid adjustable-rate mortgage averaging 3.25% this week, down from 3.27% last week and 3.76% a year ago. The one-year Treasury-indexed ARM hit 2.95% this week, down from 2.97% last week and 3.64% a year ago. Bankrate.com reported the 30-year FRM rose to 4.74% this week as concerns over the debt ceiling traveled across the financial markets, sending rates higher. That rate is up from 4.68% last week. In the Bankrate mortgage survey, the 15-year, FRM also experienced a slight uptick, growing from 3.82% last week to 3.83%. Meanwhile, the 5/1 ARM dropped from 3.36% to 3.34%, according to Bankrate. Write to Kerri Panchuk.