Malaysian authorities are looking into the “possibility” that an airliner which vanished with 239 people on board, including six Australians, had attempted to turn back, the country’s air force chief says.

Authorities also said they were expanding their search for wreckage to the west coast of Malaysia. Searches so far had concentrated on waters to the country’s east, in the South China Sea.

“There is a distinct possibility the aeroplane did a turn-back, deviating from the course,” said General Rodzali Daud, citing radar data.

“One of the possibilities is that it was returning to Kuala Lumpur.”

Malaysian Airlines flight 370 vanished early on Saturday, with the last known location of the Boeing 777-200 recorded over waters somewhere between Malaysia and southern Vietnam.

No debris has yet been found, though Vietnam has said its search planes spotted oil slicks near where contact with the plane was lost.

Rodzali said a turn-back was “corroborated by civil radar”, giving no further details.

But Malaysian Airlines CEO Ahmad Jauhari Yahya said the plane’s systems would have set off alarm bells.

“When there is an air turn-back the pilot would be unable to proceed as planned,” he said, adding authorities were “quite puzzled” over the situation.

Officials also said authorities were expanding a search operation to Malaysia’s western coast, facing the huge Indonesian island of Sumatra.

Rodzali, the air force chief, said Malaysia had “requested our Indonesian friends to assist us in that matter”.