US mortgage rates have soared this week in an unexpected reaction to the latest Treasury financial rescue plan, which has prompted investors to buy bank debt and sell bonds backed by home loans.

Interest rates on 30-year fixed-rate mortgages, as measured by Bankrate.com, rose to 6.38 per cent on Thursday from 5.87 per cent last week - before the Treasury said on Tuesday that it would take equity stakes in banks and guarantee new bank debt.