American Airlines arrival board  at O'Hare International Airport in Chicago, 10 Apr 2008

Hundreds of thousands of travelers in the United States are scrambling to reschedule flights after American Airlines canceled at least 570 more flights Friday to conduct safety inspections.

The largest U.S. airline has now grounded more than 3,000 flights since Tuesday - at least one-fifth of its fleet of jets.

The cancellations have created chaos at airports nationwide, left travelers stranded and cost American Airlines tens of millions of dollars.

The airline was forced to ground more than 300 twin-engine, medium-range jets - known as MD-80s - to conduct federally mandated safety inspections of electrical wiring.

The government's Federal Aviation Administration recently increased its scrutiny of airline safety after inspection and maintenance lapses were discovered at another carrier, Southwest Airlines.

Other U.S. airlines, including Delta, Alaska and Midwest Airlines, have grounded some aircraft this week as a result of the government audit, but the problems were most severe at American. The airline has paid for travelers' hotel stays and offered all delayed passengers a refund or a $500 coupon for future travel.

Meanwhile, Frontier Airlines announced Friday that is has filed for bankruptcy protection. Frontier says it took the action because a major credit card processor had increased the amount of money it is holding back from the carrier's ticket sales, hurting Frontier's liquidity. The low-fare carrier says it plans to keep operating while it reorganizes.

Some information for this report was provided by AFP and AP.