(CNN) -- Qantas Airways' fleet of Airbus A380 aircraft will remain grounded indefinitely after new problems with the planes' engines were uncovered over the weekend, the Australian airline's CEO said Monday.
The fleet was grounded Thursday after one of four engines on one of the airliner's A380s failed in mid-flight, forcing an emergency landing.
Qantas CEO Alan Joyce said Friday that he hoped to have the fleet up in the air within 48 hours after completing engine safety checks, but backed away from that statement Monday, telling Australian Broadcasting Corp., "We did find some issues that we are investigating further."
"Hopefully over the next day or so we will have an understanding of when the aircraft can get back in the air," he added. "Our top priority is making sure that we have the continued safe operation of the aircraft and if this takes a bit longer, it will take longer."
Thursday's incident began shortly after takeoff when the flight carrying 440 passengers and 26 crew members to Sydney, Australia, was forced to return to Singapore's Changi Airport. While above the western Indonesian island of Batam, part
of the engine's covering, or cowling, tore off.
On Friday, Joyce described the incident as an "engine issue" rather than an incident caused by poor maintenance.
He elaborated in his radio interview Monday, saying that on three of the fleet's Rolls-Royce engines "what we found is slight anomalies, oil where it shouldn't be on the engines. And we are just trying to check what could the cause of that be."
"These are new engines on new aircraft and they shouldn't have these issues at this stage and so it has given us an indication of an area for us to focus into," he said.
Rolls-Royce said in a statement last week that it has been working closely with Qantas and air safety authorities "to collect and understand information relating to the event and to determine suitable actions."
Qantas has six A380s -- the world's largest airliners -- in its fleet of 191 planes. Airbus will at some point deliver 14 more, the airline said.