Australia is investigating two objects seen on satellite images that could potentially be linked to the missing Malaysia Airlines plane, officials say.
A Norwegian ship joined planes from Australia, New Zealand and the US in searching the area 2,500km (1,550 miles) from Perth.
As night fell, the air search ended for the day, with teams saying bad weather conditions had hampered their efforts.
Flight MH370 was carrying 239 people when it disappeared on 8 March.
It was flying from Kuala Lumpur to Beijing when it lost contact with air traffic controllers and disappeared from radar.
At a news conference on Thursday, Malaysian Transport Minister Hishammuddin Hussein described the possible sighting of debris on satellite images as a "credible lead".
The largest object appeared to be 24m in size, authorities said, but warned they could be unrelated to the plane.
A number of sightings of possible debris have been investigated since the plane went missing but so far none have proved to be linked.
Prime Minister Tony Abbott announced the latest discovery based on satellite images taken on 16 March.
"The Australian Maritime Safety Authority (Amsa) has received information based on satellite information of objects possibly related to the search," Mr Abbott told parliament.
"Following specialist analysis of this satellite imagery, two possible objects related to the search have been identified."
Two Australian Orion aircraft searching the area on Thursday were joined later by aircraft from the US and New Zealand.
Amsa said the aircraft had covered an area of 23,000km² (14,000 square miles) but confirmed that they had found no debris. It said the search would resume on Friday.
The captain of the first Australian air force AP-3C Orion plane to return from the search area described the weather conditions as "extremely bad" with rough seas and high winds.
David Wright, an ABC News reporter who was on the P-8 Poseidon, said all the sophisticated plane had spotted was "a freighter and two pods of dolphins".
A Norwegian merchant ship, the St Petersburg, has also arrived in the area after responding to a shipping broadcast issued by Australia's rescue co-ordination centre.
It will be joined by an Australian naval vessel, HMAS Success, which is on its way to help with the search and Britain has also deployed HMS Echo, a coastal survey ship, to aid the operation.
The objects identified in the images were of a "reasonable size", Amsa's general manager John Young said. The largest object appeared to be about 24m (78ft) in size, he said.
"This is a lead, it is probably the best lead we have right now. But we need to get there, find them, see them, assess them, to know whether it's really meaningful or not."
He warned the objects spotted in the sea could turn out to be unrelated to MH370, such as containers that had fallen from ships.
"On this particular occasion the size and the fact that there are multiple [objects] located in the same area really makes it worth looking at," Mr Young added.
Michael Daniel, a former US Federal Aviation Administration official, told Singapore's Straits Times: "If they have a strong feeling or indication that the debris belongs to the aircraft, one of the first things authorities will do is drop sonar buoys in the water.
"If the black box is there, the buoys should be able to pick up the signals. This could take up to 48 hours but it all depends on how near or far the ships and other assets are."
Australia informed Malaysian authorities of the development on Thursday morning.
Malaysia's transport minister told reporters that while the debris sighting was a "credible lead" it needed to be verified and corroborated.
Investigators had identified two corridors of territory - one to the north and one to the south - spanning the possible positions of the plane about seven hours after take-off.
This was based on its last faint signal to a satellite - an hourly "handshake" broadcast even when the main communication systems are switched off.
Malaysia says search efforts are continuing in both corridors, involving a total of 18 ships, 29 aircraft and 6 ship-borne helicopters.
Meanwhile, relatives of those on board are still waiting for concrete news.
Bimal Sharma, a merchant navy captain whose sister Chandrika was on the plane, told the BBC he had experienced "hope and then despair and then hope and then despair".
"I have been very hopeful because it was intentionally diverted, so I don't believed it was crashed," he said. "It's been a very, very difficult time, and very emotionally stressing."
"The area where Australia is looking - I was a captain at sea - I have been through that area several times. This area has got a concentration of garbage - plastics and wood. I don't know, I don't want to believe it as yet."