By Eveline Danubrata and Nguyen Phuong Linh
KUALA LUMPUR/HO CHI MINH CITY, Vietnam (Reuters) - U .S. investigators probing the disappearance of a Malaysia Airlines passenger jet believe it may have flown for four hours after losing contact with air traffic controllers, The Wall Street Journal reported on Thursday.
If confirmed, the report would represent another dramatic twist in what is already one of the most baffling mysteries in modern aviation history - the fate of Flight MH370, which took off from Kuala Lumpur early on Saturday and dropped off civilian radar screens less than an hour into its flight to Beijing.
On the sixth day of the search, planes were sweeping an area of sea where Chinese satellite images had shown what could be debris, but had so far found no sign of the airliner.
The Wall Street Journal said U.S. aviation investigators and national security officials believed the plane flew for a total of five hours, based on data automatically downloaded and sent to the ground from the Boeing 777's engines as part of a standard monitoring programme. (http://r.reuters.com/ruw57v)
It raises the possibility that the plane, and the 239 people on board, could have flown on for an additional distance of about 2,200 miles (3,500 km), potentially reaching Pakistan, destinations in the Indian Ocean or Mongolia, the paper said.
A senior Malaysia Airlines official told Reuters that no such data existed, while a second official said he was unaware of it. A spokeswoman for engine manufacturer Rolls-Royce had no immediate comment.
Malaysia Airlines has said previously that the Rolls-Royce Trent engines stopped transmitting monitoring signals when contact with the plane was lost.
PRESSURE ON MALAYSIA
As frustration mounts over the failure to find any trace of the plane, China heaped pressure on Malaysia to improve coordination in the search. Around two-thirds of the people aboard the lost plane were Chinese.
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.
Vietnamese and Malaysian planes scanned waters where a Chinese government agency website said a satellite had photographed three "suspicious floating objects" on Sunday. The location was close to where the plane lost contact with air traffic control.
Aircraft repeatedly circled the area over the South China Sea but were unable to detect any objects, said a Reuters journalist aboard one of the planes.
One U.S. official close to the investigation said the Chinese satellite report was a "red herring".
It was the latest in a series of false signals given to the multi-national search team that has been combing 27,000 square nautical miles (93,000 square km), an area the size of Hungary, for the Boeing 777-200ER.
On Wednesday, Malaysia's air force chief said military radar had traced what could have been the jetliner to an area south of the Thai holiday island of Phuket, hundreds of miles to the west of its last known position.
His statement followed a series of conflicting accounts of the flight path of the plane, which left authorities uncertain even which sea to search.
The last definitive sighting on civilian radar screens came shortly before 1:30 a.m. on Saturday, as the plane flew northeast across the mouth of the Gulf of Thailand.
Rodzali Daud, the Malaysian air force chief, told a news conference on Wednesday that an aircraft was plotted on military radar at 2:15 a.m., 200 miles (320 km) northwest of Penang Island off Malaysia's west coast at the northern tip of the Strait of Malacca.
But there has been no confirmation that the unidentified plane was Flight MH370, Rodzali said, and Malaysia was sharing the data with international civilian and military authorities, including those from the United States.
"We are corroborating this," he added. "We are still working with the experts."
U.S. counterterrorism officials are pursuing the possibility that a pilot or someone else on board may have diverted the plane toward an undisclosed location after intentionally turning off the jetliner's transponders to avoid radar detection, The Wall Street Journal said, citing one person tracking the probe.
If the military radar signal cited by Rodzali was the missing plane, the aircraft would have flown for 45 minutes and dropped only about 5,000 feet (1,500 metres) in altitude since its sighting on civilian radar in the Gulf of Thailand.
That would mean the plane had turned sharply west from its original course, travelling hundreds of miles over the Malay Peninsula from the Gulf of Thailand to the Andaman Sea to a point roughly south of Phuket and east of the tip of Indonesia's Aceh province and India's Nicobar island chain.
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.
U.S. EXPERTS ASSISTING
The U.S. National Transportation Safety Board said in a statement that its experts in air traffic control and radar who travelled to Kuala Lumpur over the weekend were giving the Malaysians technical help.
A U.S. official in Washington said the experts were shown two sets of radar records, military and civilian, and they both appeared to show the plane turning to the west across the Malay peninsula.
But the official stressed the records were raw data returns that were not definitive.
A dozen countries are taking part in the search, with 42 ships and 39 aircraft involved.
Authorities have not ruled out any cause for the disappearance. 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 men 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.
Boeing has declined to comment beyond a brief statement saying it was monitoring the situation.