* "Pings" indicate missing plane's systems on for some time
* China's premier demands "relevant party" step up coordination
* 227 passengers and 12 crew aboard lost flight
* Area being searched the size of Hungary
By Anshuman Daga and Mark Hosenball
KUALA LUMPUR/WASHINGTON, March 14 (Reuters) - Satellites picked up faint electronic pulses from Malaysia Airlines Flight 370 after it went missing on Saturday, but the signals gave no information about where the stray jet was heading and little else about its fate, two sources close to the investigation said on Thursday.
But the "pings" indicated that the aircraft's maintenance troubleshooting systems were switched on and ready to communicate with satellites, showing the aircraft, with 239 people on board, was at least capable of communicating after the jet lost touch with Malaysian air traffic controllers.
The system transmits such pings about once an hour, according to the sources, who said five or six were heard. However, the pings alone are not proof that the plane was in the air or on the ground, the sources said.
An international search is under way over a vast area in the Gulf of Thailand, the Andaman Sea and on both sides of the Malay Peninsula. The United States, which has sent ships and planes, said the area may be expanding 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," White House spokesman Jay Carney told reporters in Washington.
U.S. defense officials later told Reuters that the U.S. Navy guided-missile destroyer, USS Kidd, was en route to Strait of Malacca, west of the Malaysian peninsula, to continue the search for the missing jet, answering a request from the Malaysian government. The Kidd had been searching the areas south of the Gulf of Thailand, along with the destroyer USS Pinckney.
A U.S. defense official said a Navy P-3 Orion aircraft had already searched the Strait of Malacca.
LITTLE FRESH LIGHT
The new information shed little light on the mystery of what happened to the plane, whether there was a technical failure, a hijacking or another kind of incident on board after it took off from Kuala Lumpur en route to Beijing.
While the troubleshooting systems were functioning, no data links were opened, the sources said, because the companies involved had not subscribed to that level of service from the satellite operator, the sources said.
Boeing Co, which made the missing 777 airliner, and Rolls-Royce, which supplied its Trent engines, declined to comment.
Earlier Malaysian officials denied reports that the aircraft had continued to send technical data and said there was no evidence that it flew for hours after losing contact with air traffic controllers early Saturday.
The Wall Street Journal had reported that U.S. aviation investigators and national security officials believed the Boeing 777 flew for a total of five hours, based on data automatically downloaded and sent to the ground from its engines as part of a standard monitoring program.
Malaysian authorities have said the last civilian contact occurred as the Boeing 777-200ER flew north into the Gulf of Thailand. They said military radar sightings indicated the plane may have turned sharply to the west and crossed the Malay Peninsula toward the Andaman Sea.
It is one of the most baffling mysteries in the history of modern aviation - there has been no trace of the plane since nor any sign of wreckage despite a search by the navies and military aircraft of over a dozen countries across Southeast Asia.
"It's extraordinary that with all the (satellite and telecommunication) technology that we've got that an aircraft can disappear like this," Tony Tyler, the head of the International Air Transport Association that links over 90 percent of the world's airlines, told reporters in London.
On the sixth day of the search, planes scanned an area of sea where Chinese satellite images had shown what could be debris but found no sign of the airliner.
Malaysian Transport Minister Hishammuddin Hussein told a news conference the images were provided accidentally, saying the Chinese government neither authorised nor endorsed putting them on a website. "The image is not confirmed to be connected to the plane," he said.
It was the latest in a series of contradictory reports, adding to the confusion and agony of the relatives of the passengers, about two-thirds of whom were Chinese.
As frustration mounted over the failure to find any trace of the plane, China heaped pressure on Malaysia to improve coordination in the search.
Premier Li Keqiang, speaking at a news conference in Beijing, demanded that the "relevant party" step up coordination while China's civil aviation chief said he wanted a "smoother" flow of information from Malaysia, which has come under heavy criticism for its handling of the disaster.
Indonesia and Thailand have said their militaries detected no sign of any unusual aircraft in their airspace. Malaysia has asked India for help in tracing the aircraft and New Delhi's coastguard planes have joined the search.
Malaysian police have said they were investigating whether any passengers or crew on the plane had personal or psychological problems that might shed light on the mystery, along with the possibility of a hijacking, sabotage or mechanical failure.
Two of the passengers on board were discovered by investigators to have false passports, but they were apparently seeking to emigrate illegally to the West.
The Boeing 777 has one of the best safety records of any commercial aircraft in service. Its only previous fatal crash came on July 6 last year when Asiana Airlines Flight 214 struck a seawall with its undercarriage on landing in San Francisco, killing three people.