Australian controllers of the search for the missing Malaysian Airlines passenger jet have discounted the southern Indian Ocean zone as the plane's final resting place.
Flight MH370 went missing on 8 March this year as it flew from Kuala Lumpur to Beijing.
Using satellite data, officials have concluded that the airliner, which had 239 people on board, ended its journey in the Indian Ocean, north-west of the Australian city of Perth.
But no trace of the Boeing 777 has been found, nor any reason for its disappearance.
The Joint Agency Coordination Centre said on Thursday the Bluefin-21 submersible robot has completed its last mission searching in the vicinity of acoustic signals detected in April and found nothing.
Efforts would now focus on reviewing search data, surveying the sea floor and bringing in specialist equipment, the BBC reports.
Meanwhile, a United States Navy official says four 'pings' heard by an acoustic locator are more likely to have come from other ships or the towed locator, not from the plane's "black box" recorders.
The 'pings' prompted a multi-national search that has covered 4.64 million square kilometres of ocean, without success.
Australia's Deputy Prime Minister Warren Truss says new analysis of all existing data suggests a massively expanded search zone up to 800km long and 70km wide and search efforts would continue.