After checking dozens of Boeing 737 aircraft, officials of Australian national airline Qantas said cracks found in three jets will need more "complex "repairs than first thought.

Qantas has taken the three 737s out of service for repairs and is checking dozens of others for safety.

Last week engineers for the airline found cracks in a part called the "pickle fork" used to strengthen the connection between the wing and the body of the plane.

The head of engineering for Qantas, Chris Snook, said repairs would be "complex".

"The airplane will be on the ground for a month ... Boeing have done a repair scheme trial in California," he told the Australian Broadcasting Corporation.

"It's taken 11 days to do the pickle fork in-and-out, but it took about a month to do the entire process."

The US Federal Aviation Administration ordered inspections of Boeing 737 aircraft in September after the company told the agency that structural cracks had been found on heavily used planes.

Qantas said in a statement it has been liaising with Boeing and safety regulators in Australia "on an issue affecting some of the worldwide fleet of 737s".

"We would never operate an aircraft unless it was completely safe to do so. Even when a crack is present, it does not immediately compromise the safety of the aircraft."

The statement added: "Qantas rejects the alarmist claims made by the licensed engineers' union, which are irresponsible and completely inconsistent with advice from regulators and the manufacturer."

This was aimed at the Australian Licensed Aircraft Engineers Association, which has called on the airline to ground all 75 aircraft in its 737 fleet.

Qantas's Boeing 737s are used on many domestic routes and selected overseas services to New Zealand, Indonesia and Fiji.

Qantas fleet safety captain Debbie Slade said that she understood how the word "crack" could be concerning to passengers.

"What is not clearly understood is that all mechanical devices such as cars, bridges, ships and airplanes all have structural cracks," said John Page, an aircraft designer and senior lecturer in aerospace engineering in the School of Mechanical and Manufacturing Engineering at the University of New South Wales.

"The real question is how design engineers manage these phenomena," he told China Daily.

"In the case of aircraft, the engineer designs the structure to ensure a crack is detected before it becomes critical. To ensure this occurs an inspection procedure is designed to make sure the crack is found before it becomes critical."

He said the significant factor is the rate at which the crack grows.

"Usually this is easy to calculate. If the crack might grow such that it could lead to a catastrophic failure, then it has to be remediated before continued operation," he said.

"The detection of not-critical cracks means the safety system is working. What we do not want to see is a failure with no warning," he added.