Australian authorities are quick to warn that debris found floating in the southern Indian Ocean may not belong to missing flight MH370.

The global search for missing Malaysia Airlines flight MH370 has its best new lead, with possible debris spotted in water west of Australia.

Military aircraft and merchant ships are racing to a position in the southern Indian Ocean about 2500 kilometres southwest of Perth, where a satellite identified two floating objects.

One measured about 24 metres, while the other was smaller.

Australian authorities say they are possible remnants of the Boeing 777 that went missing on a March 8 flight from Kuala Lumpur to Beijing, carrying 239 people, including six Australians and two New Zealanders.

But John Young of the Australian Maritime Safety Authority (AMSA) was cautious not to raise hopes, saying the satellite imagery shows “a sort of blob” with no features to distinguish it as aircraft fragments.

“It’s probably the best lead we have right now, but we have to get there, find them, see them, assess them to know whether it’s really meaningful or not,” the emergency response division manager said.

Water in the area is thousands of metres deep and searchers are battling poor visibility, with last light due about midnight (AEDT) on Thursday.

“Every lead is a hope,” Malaysian Transport Minister Hishammuddin Hussein told reporters in Kuala Lumpur.

“This time I just hope that it is a positive development.”

Mr Hussein said the Australian finding will not detract from a widespread international search, which continues combing areas identified by experts as being possible locations for the jet, based on MH370’s fuel range.

Altogether there are 18 ships, 29 aircraft and six ship-borne helicopters working on the operation.

“Until we are certain we have found MH370, search and rescue operations will continue,” he said.

Australia has been co-ordinating the search operation in the southern Indian Ocean.

If the debris belongs to the aircraft, it indicates MH370 ended up thousands of kilometres from its planned destination, raising further questions about why it changed course.

But the priority for AMSA remains identifying the bobbing objects.

It was not uncommon to find floating debris, including shipping containers that had been washed overboard, Mr Young said.

“On this particular occasion, the size and the fact that there are a number located in the sea at the same area really makes it worth looking at.”

Authorities should know something definite about the objects within “two or three days”, Australian Defence Minister David Johnston says.

Australia’s defence forces were doing everything they could in one of the most remote locations in the world, Senator Johnston told AAP.

An RAAF C-130 Hercules has dropped marker buoys at the location identified by satellite, and military aircraft from Australia, New Zealand and the United States are combing the area.

A merchant ship was also headed to the area.

“(The objects) will be difficult to find. They might not be associated with the aircraft and we have plenty of experience of that in other searches,” Mr Young said.

The search area is a long way from the Australian mainland and once aircraft reach the location, they have about only two hours of fuel before having to return to base.

Asked about his message to the family and friends of people on board flight MH370, Mr Young said Australia would continue to search until it found something.

“AMSA is doing its level best to find anyone who may have survived,” he said.

Australia is sharing its information with 25 other countries involved in the search operation, and Prime Minister Tony Abbott confirmed on Thursday he had spoken to his Malaysian counterpart about the latest update.

Unfavourable weather is limiting visibility, which authorities say might hinder both air and satellite search efforts.