Update 10.03pm: Malaysian Prime Minister Najib Razak says "with great sadness" data now confirmed that the aircraft's last position was in the Indian Ocean west of Perth.
Just before Mr Razak's media conference in Kuala Lumpur tonight Malaysia Airlines sent a text messages in English to the relatives of people on the missing plane saying: "Malaysia Airlines deeply regrets that we have to assume beyond any reasonable doubt that MH370 has been lost and that none of those on board survived.
"As you will hear in the next hour from Malaysia's Prime Minister, we must now accept all evidence suggests the plane went down in the Southern Indian Ocean."
Mr Razak told the gathered media: "This evening I was briefed by representatives from the UK Air Accidents Investigation Branch (AAIB). They informed me that Inmarsat, the UK company that provided the satellite data which indicated the northern and southern corridors, has been performing further calculations on the data.
"Using a type of analysis never before used in an investigation of this sort, they have been able to shed more light on MH370's flight path.
"Based on their new analysis, Inmarsat and the AAIB have concluded that MH370 flew along the southern corridor, and that its last position was in the middle of the Indian Ocean, west of Perth.
"This is a remote location, far from any possible landing sites. It is therefore with deep sadness and regret that I must inform you that, according to this new data, flight MH370 ended in the southern Indian Ocean."
Earlier Prime Minister Tony Abbott says an RAAF P3 Orion located two objects in the search for missing Malaysian airlines flight MH370.
The objects were sighted at about 11.45pm WST about 2500km south-west of Perth.
“The crew on board the Orion reported seeing two objects, the first a grey or green circular object and the second an orange rectangular object,” he told parliament this evening.
The HMAS Success is in the area and attempting to recover the objects, and a US Navy P8 Poseidon, a second RAAF Orion and a Japanese Orion are also en route to try to find the items.
“It is possible that the objects could be received within the next few hours, or by tomorrow morning at the latest,” Malaysian Defence Minister Hishammuddin Hussein told a press conference in Malaysian capital Kuala Lumpur.
They are different to the pieces spotted by a Chinese plane earlier in the day.
“I caution again ... that we don’t know whether any of these objects are from MH370, they could be flotsom,” Mr Abbott told parliament.
“Nevertheless we are hopeful that we can recover these objects soon and they will take us a step closer to resolving this tragic mystery.”
Chinese aircrew have spotted “suspicious objects” in the southern Indian Ocean earlier today, the official Xinhua news agency says.
The “white and square” objects were spotted by searchers aboard a Chinese Ilyushin-76 plane, it said on Monday.
“The crew has reported the co-ordinates - 95.1113 degrees east and 42.5453 south - to the Australian command centre as well as Chinese icebreaker Xuelong, which is en route to the sea area,” Xinhua said.
Earlier Xinhua reports said a Chinese military plane set off on Monday morning from Perth to find the “suspicious debris” captured by satellite imagery in the remote waters.
The US Navy is sending a black box locator to the area of the southern Indian Ocean being scoured for the missing Malaysia Airlines jet, following a cluster of weekend debris sightings.
The navy called the move a “precautionary measure” in case those sightings confirm the location of the aircraft which disappeared on March 8, with 239 people on board.
“If a debris field is confirmed, The Navy’s Towed Pinger Locator 25 will add a significant advantage in locating the missing Malaysian aircraft’s black box,” Commander William Marks, a spokesman for the US Seventh Fleet, said in an email.
The locator system relies on acoustic signals to help find flight recorders - also known as black boxes - on downed navy and commercial aircraft to a maximum depth of 6,000m, he added.
However, the statement cautioned that the deployment did not mean the missing jet’s location had been confirmed.
“Please note that movement of the Towed Pinger Locator into the region is not an indication that we have confirmed a debris field,” it said.
“It’s a precautionary measure so that if we do find debris, we’ll be ready to deploy the equipment to listen for the black box.”
The sighting of a wooden pallet and other debris that may be linked to the Malaysia Airlines passenger jet gave the sense on Sunday that the hunt was finally on the right track after more than two weeks of false leads and dead ends.
It was reinforced by new French satellite data indicating floating objects in the southern search area.
Australian officials said the pallet, along with belts or straps, was spotted on Saturday in a remote stretch of the Indian Ocean that has become the focus of the search - around 2,500km south-west of Perth.