The US Navy has ordered a ship to the Indian Ocean to search for a missing Malaysian airliner amid reports the plane kept "pinging" a satellite after losing radar contact.
The focus of search efforts shifted from the South China Sea after the White House said "new information" indicated the plane may have gone down to the west in the Indian Ocean.
"The USS Kidd is transiting the Strait of Malacca en route to the Indian Ocean," a navy official told AFP, referring to a guided-missile destroyer initially deployed to the Gulf of Thailand.
An additional US aircraft, a P-8 Poseidon surveillance plane, also was headed to the area, where a P-3 Orion was already aiding the search effort, said the official, who spoke on condition of anonymity.
The move followed reports that the Malaysia Airlines Boeing 777's communication system continued to "ping" a satellite for a number of hours after the plane disappeared off radar.
The signal came from the jet's "airplane health management system" that provides a flow of data on the airliner's operations, according to the Wall Street Journal and ABC News.
The Journal later retracted one detail in its original report, which had incorrectly stated that investigators were looking at signals from the plane's Rolls-Royce engines.
A separate ABC news report, citing two anonymous US officials, said investigators now believe the aircraft's data reporting system and its transponder - which reports its position in flight - shut down separately.
The fact that the devices appear to have been shut off at a 14-minute interval from one another suggests that they may have been deliberately disabled or at any rate did not fail as a result of a single catastrophic airframe failure, the report said.
A string of previous clues about Flight MH370 have led nowhere.
"This situation is unprecedented. MH370 went completely silent over the open ocean," said Malaysia's acting Transport Minister Hishammuddin Hussein.
"This is a crisis situation. It is a very complex operation, and it is not obviously easy. We are devoting all our energies to the task at hand."
How do the data signals work?
Boeing offers a satellite service that can receive a stream of data during flight on how the aircraft is functioning and relay the information to the plane's home base. The idea is to provide information before the plane lands on whether maintenance work or repairs are needed.
Malaysia Airlines didn't subscribe to that service, but the plane still had the capability to connect with the satellite and was automatically sending pings, the official said.
"It's like when your cellphone is off but it still sends out a little 'I'm here' message to the cellphone network," the official said. "That's how sometimes they can triangulate your position even though you're not calling because the phone every so often sends out a little bleep. That's sort of what this thing was doing."
The plane had enough fuel to fly about four more hours, the US official said.
Boeing did not comment.
Messages involving a different, more rudimentary data service also were received from the airliner for a short time after the plane's transponder a device used to identify the plane to radar went silent, the official said.
If the plane had disintegrated during flight or had suffered some other catastrophic failure, all signals the pings to the satellite, the data messages and the transponder would be expected to stop at the same time.
One part of the hunt is in the South China Sea, where the aircraft was seen on civilian radar flying northeast before vanishing without any indication of technical problems. A similar-sized search is also being conducted in the Strait of Malacca because of military radar sightings that might indicate the plane turned in that direction after its last contact, passing over the Malay Peninsula.
The total search area being covered is about 92,600 square kilometres - about the size of Portugal.
Asked if it were possible that the plane kept flying for several hours, Hishammuddin said: "Of course. We can't rule anything out. This is why we have extended the search. We are expanding our search into the Andaman Sea." The sea is northwest of the Malay Peninsula.
He said Malaysia was asking for radar data from India and other neighbouring countries to see if they can trace it flying northwest. India says its navy, air force and coast guard will search for the plane in the south Andaman Sea.
White House spokesman Jay Carney told reporters on Thursday that unspecified "new information" had led to the search being expanded into the Indian Ocean.
"It's my understanding that based on some new information that's not necessarily conclusive, but new information, an additional search area may be opened in the Indian Ocean," he said.
"We are consulting with international partners about the appropriate assets to deploy."
The US Navy 7th Fleet said it is moving one of its ships, the USS Kidd, into the Strait of Malacca, west of Malaysia.
In the latest disappointment, search planes failed to find any debris from the plane after they were sent Thursday to an area of the South China Sea off the southern tip of Vietnam, where satellite images published on a Chinese government website reportedly showed three suspected floating objects.
"There is nothing. We went there. There is nothing," Hishammuddin said.
More than two-thirds of those on Flight MH370 were from China, which has shown impatience with the absence of any results. Chinese Premier Li Keqiang said in Beijing he would like to see better coordination among countries in the search.
The passengers' "families and friends are burning with anxiety. The Chinese government and Chinese people are all deeply concerned about their safety," he said at the close of the annual session of the country's legislature. "As long as there is a glimmer of hope, we will not stop searching for the plane."
He said China had eight ships and 10 satellites searching for the plane.
Malaysia has been criticised for its handling of the search, in part because it took several days to fully explain why it couldn't say whether the plane had turned back. Officials say they are not hiding anything and are searching areas where the plane is most likely to be, while trying to establish its actual location.
The clues that delivered false hope
Besides the Chinese satellite photos and the so-far fruitless search based on the possible sighting on military radar, there have been other developments in the aviation mystery that have failed to lead to finding the plane or the cause of its disappearance:
Oil slicks seen Saturday were found to have nothing to do with the jetliner.
A yellow object spotted by a search plane turned out to be ordinary sea trash.
Officials initially said four or five passengers checked in for the flight but did not board, fueling speculation about terrorism. Officials later said some people with reservations never checked in and were simply replaced by standby passengers, and no baggage was removed.
Officials said two men, later identified as Iranians, boarded the plane with stolen passports. It was later reported that they were unlikely to be linked to terrorist groups.
Investigators have not ruled out any possible cause for the plane's disappearance.
Experts say a massive failure knocking out electrical systems, while unlikely, could explain why the transponders were not working. Another possibility is that the pilot, or a passenger, likely one with some technical knowledge, switched off the transponders in the hope of flying undetected.
"There is no real precedent for a situation like this. The plane just vanished," Hishammuddin said.
Experts say that if the plane crashed into the ocean, some debris should be floating even if most of the jet is submerged. Past experience shows that finding the wreckage can take weeks or even longer, especially if the location of the plane is in doubt.
From Herald reporter Lincoln Tan in Malaysia:
Frustration and exasperation best describe the mood in Kuala Lumpur. Six days of confusion and contradictory information over the missing Malaysia Airlines Flight MH370 from Malaysian authorities is more than some can take.
Even the usually more compliant local journalists are turning against the authorities, calling on officials to be more upfront and transparent at press conferences. Yesterday, I was issued a warning that I would be removed from the hotel where relatives of missing Chinese passengers stayed if I tried to interview someone who had a day earlier agreed to speak to me.
A government official also tried to tell journalists who they could and could not interview at a press conference when some spoke to the Chinese ambassador.
Families of passengers are distraught over the limited and conflicting answers they are getting from government officials.
One even went so far as to tell me Malaysia cannot be trusted and this was making her "go crazy".
Malaysia is aiming to be a developed nation by 2020, but here, the feeling is that it cannot manage a crisis of this scale.
When the story first broke, there was much sympathy for Malaysia, but this mess is resulting in anger and angst.
As we enter day seven of the search efforts that are mired in confusion, it is time the government got its act together.
All eyes are on Malaysia, and failure to manage this is not a good look for a country that's promoting 2014 as Visit Malaysia Year.
Besides, it owes it to the families of those missing on Flight MH370 - including two from New Zealand.