China's Premier Li Keqiang has pledged to continue search efforts for the missing Malaysia Airlines flight "as long as there is a glimmer of hope".
His comments came as Vietnam and Malaysia investigated satellite images released by China that appeared to show possible debris in the South China Sea.
However, they were unable to find debris at the same location.
Meanwhile, one report said US officials suspect the plane flew for four hours after its last confirmed location.
The report, if true, would mean the plane could have flown much further, and be well out of current publicised search areas.
Flight MH370 from Kuala Lumpur to Beijing went missing late on Friday, with 239 passengers and crew on board.
Teams have since been searching waters on both sides of the Malaysian peninsula for any sign of the plane.
Satellite images, released by China on Wednesday, appeared to identify floating objects 150 miles (250km) or so from the aircraft's last known position.However, Dinh Viet Thang, deputy director of Vietnam's Civil Aviation Authority, told AFP that two planes sent to inspect the area had returned, and "found nothing so far".
Similarly, Malaysian Civil Aviation Chief Azharuddin Abdul Rahman told reporters that Malaysia's planes did not find anything.
"There is nothing. We went there, there is nothing," he said.
Several previous sightings of possible debris in other areas have proved fruitless.
Diverted plane?Meanwhile, the Wall Street Journal said that US investigators suspected that the plane stayed airborne long after its last confirmed contact.
Citing two unidentified sources "familiar with the details", WSJ said that data downloaded from the Boeing 777's engines led investigators to believe that the plane flew for a total of five hours.
US officials are exploring whether someone on the plane could have turned off the aircraft's transponders to avoid radar detection, and then diverted the plane towards another location, WSJ added.
With 154 Chinese citizens on board the missing jet, China has redeployed at least 10 of its satellites to help in the search.
The photographs of the three objects, the largest of which measures around 24 metres by 22 metres, were taken on Sunday, the day after the plane vanished, but were only published on Wednesday.
The wingspan of the missing Boeing 777 is around 61 metres. The satellite images place the objects 150 miles (250km) or so from the aircraft's last known position over the South China Sea and 250 miles from an oil rig on which a worker reported seeing a burning object in the sky on Saturday morning.
Malaysian Airline and Boeing officials, as well as representatives from plane engine-makers Rolls-Royce, declined to comment on the points reported by WSJ.Chinese Premier Li Keqiang made his comments at a press conference that came as China's annual parliament session ended: "We will not give up on any suspected clue that has been found.
"This is an international and large-scale search operation involving many countries."
He also urged Malaysia to step up search efforts, saying: "The Chinese government has asked relevant parties to enhance co-ordination, investigate the cause, locate the missing plane as quickly as possible and properly handle all related matters."
Earlier, China's foreign ministry complained that there was "too much confusion" regarding information released about the plane's flight path.
The plane vanished late on Friday, about an hour after it took off from Kuala Lumpur.
No distress signal or message was sent from the flight, which was carrying 153 Chinese nationals among its passengers.
Earlier, Malaysian authorities revealed that the last communication from the jet suggested everything was normal on board.
Flight MH370 replied "All right, roger that" to a radio message from Malaysian air control, authorities said. Minutes later all contact with the plane was lost.