A gauge of U.S. corporate credit risk rose for a third day, reaching the highest in six months, as concern mounts that the Federal Reserve will start scaling back record bond-buying that has bolstered debt markets.

The Markit CDX North American Investment Grade Index, a credit-default swaps benchmark that investors use to hedge against losses or to speculate on creditworthiness, increased 1.8 basis points to a mid-price of 94.9 basis points at 12:34 p.m. in New York, according to prices compiled by Bloomberg. The gauge, trading at the highest level since December, has surged 10.8 basis points since June 14, heading for its biggest weekly increase since May 2012, excluding rolls into new series.

Investors' confidence in corporate debt has plunged since Fed Chairman Ben S. Bernanke said June 19 that the central bank may pare $85 billion of monthly bond purchases this year and end it in mid-2014 if the economy achieves the Fed's objectives. The Fed's stimulus measures, known as quantitative easing, have suppressed interest rates, pushing investors into riskier assets such as corporate bonds in search of higher yields.