Qantas aircraft are seen on the tarmac at Melbourne International Airport in Melbourne, Australia. (REUTERS/Phil Noble)

"Of the 33 of Qantas' 737 aircraft that required inspection, three were found to have a hairline crack in the pickle fork structure," the airline said in a statement, referring to a part which helps bind the wing to the fuselage.

"These aircraft have been removed from service for repair," it said.

Boeing announced on Thursday that dozens of its popular 737NG planes had been taken out of service after cracks in them were detected.

US regulators earlier this month ordered inspections of older NG aircraft, directing planes with more flying hours to be checked within seven days.

A Boeing spokesperson on Thursday told AFP in Sydney that less than 5 per cent of 1,000 planes were found to have cracks and were grounded for repair.

South Korea's Korean Air and US carrier Southwest Airlines have also taken some of their 737NGs out of service after cracks were discovered in the pickle forks, while several other leading airlines said inspections had not turned up cracks on their aircraft.

A Singapore Airlines spokesperson told CNA on Friday that SilkAir's Boeing 737NG fleet was not affected.

The new difficulties compound the troubles facing the US manufacturer, which has faced tumbling profits, federal scrutiny and calls for its CEO to resign after deadly crashes involving the 737 MAX, the successor aircraft for the 737NG.

Boeing and Qantas stressed travellers should not be concerned by the issues with the 737NGs.

"We would never fly an aircraft that wasn't safe," said Andrew David, the CEO of Qantas Domestic.

"Even where these hairline cracks are present they're not an immediate risk, which is clear from the fact the checks were not required for at least seven months."

VNF/AFP