SYDNEY, AUSTRALIA - Australia's biggest airline Qantas said Wednesday it had grounded six of its Boeing 737-400 aircraft because of irregularities in their paperwork.

The airline, which has suffered a string of recent problems including a mid-air blast which ripped a hole in a plane, said it needed to cross-check maintenance records against work carried out in Australia.

The issue did not have any safety implications, Qantas Engineering's executive general manager David Cox said in a statement.

'Until we are absolutely certain that everything is 100 percent correct we cannot operate the aircraft,' he told the Australian Broadcasting Corporation.

'What we picked up is a step in the maintenance check which does not look like it has been completed, and that's why we've got to go back and verify that.'

Cox said Australia's Civil Aviation Safety Authority (CASA) had been advised of the problem.

CASA is conducting a safety review of Qantas over the recent incidents, which include a Boeing 747-400 making an emergency landing in Manila in July after a blast, believed to have been caused by an exploding oxygen bottle,
ripped a hole in its fuselage.

On July 28, a Qantas 737-800 returned to Adelaide after a landing gear door failed to retract and earlier this month a Boeing 767 bound for Manila turned back to Sydney after developing a leak of hydraulic fluid.

Last week a Qantas jet was grounded because of an air-conditioning fault.