SYDNEY - QANTAS extended the grounding of its fleet of Airbus A380 superjumbos on Monday after uncovering 'slight anomalies' in some engines after a mid-air blow-out caused a major scare last week.

Chief executive Alan Joyce said rigorous testing had found leaking oil in three of the Rolls-Royce engines, pushing back the long-haul planes' return to action by some days.

'On three of the engines what we found is slight anomalies - oil where oil shouldn't be on the engines,' Mr Joyce told Australian public broadcaster ABC. 'These are new engines on new aircraft and they shouldn't have these issues at this stage, so it's given us indication of an area for us to focus into.

'We are keeping an open mind on it but... we think it could have been a materials issue on the engine or a design issue on the engine.' Qantas' impeccable safety record - the Australian flag-carrier has never had a fatal jet crash in 90 years - has come into question after two engine explosions in as many days.

On Thursday, a Qantas Airbus A380 was forced to return to Singapore for an emergency landing after one of its four Rolls-Royce Trent 900 engines exploded just minutes into its flight, showering an Indonesian town with debris.

And on Friday, a Qantas Boeing 747 - which was carrying the A380's captain - also had to turn back to Singapore after another model of Rolls-Royce engine also failed in mid-air.

'Anomalies' found in engines