Three US airlines have seen their senior debt ratings cut to junk status after Standard & Poor's (S&P), the rating agency, downgraded nine airlines across the sector.

American Airlines, AMR Corp and Delta Airlines, whose ratings had been hovering on the BBB- cusp of investment and sub-investment grade, fell to BB on Thursday, when S&P also cut the credit ratings of AirTran Airways, Alaska Air, Amtran, Atlantic Coast Airlines, Continental and Delta.