KUALA LUMPUR, Malaysia — A Malaysia Airlines plane sent signals to a satellite for four hours after the aircraft went missing, an indication that it was still flying for hundreds of miles or more, a US official briefed on the search said Thursday.
Six days after the plane with 239 people aboard disappeared, Malaysian authorities expanded their search westward toward India, saying the aircraft may have flown for several hours after its last contact with the ground shortly after takeoff from Kuala Lumpur for Beijing.
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.’’
The US official, who spoke on condition of anonymity because he wasn’t authorized to discuss the situation by name, said the Boeing 777-200 wasn’t transmitting data to the satellite, but was instead sending out a signal to establish contact.
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 35,800 square miles — 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 neighboring 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. The US Navy Seventh 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 floating objects.