American Airlines, SouthWest, United and Delta planes were among airlines over which there was a string of maintenance concerns and canceled flights.

Four United States airlines are being investigated for failing to comply with federal aviation regulations.

US Federal Aviation Administration officials on Wednesday said an audit of airline maintenance records found three of the four carriers had missed inspection deadlines.

Wiring in the wheel well was one area where carriers failed to comply with 'airworthiness directives', the officials said.

One carrier had failed to submit a required plan and the other three airlines were found to have aircraft overdue for inspections.

While the agency would not name the carriers under investigation, it said penalties could be levied. It will be several months before the probes are complete.

Acting FAA Administrator Robert A. Sturgell said the flying public should feel safe despite a recent string of maintenance concerns and cancelled flights involving Southwest Airlines Co, AMR Corp's American Airlines, Delta Air Lines Inc and UAL Corp's United Airlines.

He emphasised that his agency's review of nearly 2,400 airline maintenance records found US carriers were in compliance with federal regulations nearly 99 per cent of the time.

"The bottom line is ... flying is safer today than at any time in the past," Sturgell said at a briefing at Washington's Reagan National Airport.

"It's no accident or miracle."

Still, the FAA is taking new action both to make it easier for inspectors to raise concerns and to strengthen ethics policies aimed at easing potential conflicts of interests.

The FAA will launch a new reporting system by the end of this month to provide employees an additional way to raise safety concerns they feel are not receiving the necessary attention or response from management, Sturgell said.

And by June 30, the agency will start a rule-making process to set a two-year 'cooling off' period for former inspectors hired by airlines to match the time that new inspectors hired from industry must wait before they can oversee their former employer.