Investigators looking into what forced flight QF30, on its way to Melbourne, into an emergency landing in Manila last Friday - with a 3m rip in its fuselage - are focusing on two oxygen cylinders normally located in that area.

Australian investigators said one bottle was missing.

'It is too early to say whether this was the cause of the explosion,' said investigator Neville Blyth from the Australian Transport Safety Bureau (ATSB).

'But one of the cylinders which provide back-up oxygen is missing.'

If confirmed, the finding would have implications for other 747 planes around the world, Civil Aviation Safety Authority (Casa) spokesman Peter Gibson told Agence France-Presse in Sydney.

Yesterday, Qantas was ordered to check all oxygen bottles on its fleet of Boeing 747s, while six separate investigation teams began searching for the cause of the mid-air drama, which potentially threatens the future of more than 1,300 jumbos around the world.

Casa said Boeing had put all airlines flying its planes on notice, in case the investigation unearthed a problem that affected all 747s.

'It has become an international investigation,' Mr Gibson said, according to Australia's Sunday Telegraph.

'We've set up a team of engineers and other experts to start reviewing all the information coming from Manila, so we can respond quickly if we need to, in terms of other action that may need to be taken.'

The ATSB is leading the probe. The United States Federal Aviation Administration is also involved, along with Boeing.

Mr Blyth said investigators had ruled out terrorism.

'There is no evidence of a security-related event here. Philippine sniffer dogs have inspected the baggage and found no materials of concern.'

Earlier, Mr Gibson discounted a report that corrosion was to blame for the incident, saying that while minor corrosion had been found in the plane during a routine check a few months ago, it was in a totally different part of the aircraft.

The plane was flying from Hong Kong to Melbourne last Friday when an explosive bang led to a sudden loss of cabin air pressure.

It plunged 6,000m before stabilising and then made an emergency landing in Manila. Sources close to the investigation said the 365 passengers and 19 crew members, who returned to Melbourne last Saturday, were lucky to be alive.

The pilot, Mr John Bartels, said that training enabled the flight crew to handle the emergency. 'There were three of us in the cockpit and we all worked together and focused on doing what we had to do to get the aircraft down safely, which is exactly what we are trained to do.'

Passengers praised the crew's handling of the incident, but some said that not all the oxygen masks worked properly.

'Ours didn't come down, and my husband just about (passed out) because he didn't have any oxygen for about three minutes,' passenger Beverley Doors told Australian radio yesterday.

Passenger David Saunders said one man in front of him smashed the ceiling panel to force his mask to drop, and that children were screaming and flailing.

'Their cheeks and lips were turning blue from lack of oxygen,' he said.

Qantas strongly defended its procedures, saying the masks had been properly inspected, but suggested aircraft systems may have been damaged in the explosion.

'Qantas regularly inspects all oxygen masks in line with the manufacturer's instructions and there was nothing to suggest the masks were not in perfect working order for the flight,' it said.

REUTERS, ASSOCIATED PRESS, AGENCE FRANCE-PRESSE