* Air force chief denies saying plane tracked to Malacca Strait
* Source had said radar suggested plane turned back and flew some distance
* Hijack, sabotage, mechanical failure also being investigated
* Still no sign of missing plane, search in fifth day
* 227 passengers and 12 crew aboard lost flight
By Stuart Grudgings and Nguyen Phuong Linh
KUALA LUMPUR/PHU QUOC, Vietnam, March 12 (Reuters) - The search for a missing Malaysia Airlines jetliner expanded on Wednesday to cover a swathe of Southeast Asia, from the South China Sea to India's territorial waters, with authorities no closer to explaining what happened to the plane or the 239 people on board.
Vietnam briefly scaled down search operations in waters off its southern coast, saying it was receiving scanty and confusing information from Malaysia over where the aircraft may have headed after it lost contact with air traffic control.
Hanoi later said the search - now in its fifth day - was back on in full force and was even extending on to land. China also said its air force would sweep areas in the sea, clarifying however that no searches over land were planned.
The seas off India's Andaman and Nicobar Islands are also being combed for traces of the lost jet.
"We are expanding to the east of the expected route of the flight and on land," Lieutenant General Vo Van Tuan, Vietnam's deputy army chief of staff and spokesman for its search and rescue committee, told reporters.
The confusion over where to look is adding to one of the most baffling mysteries in modern aviation history, and prolonging the agonising wait for hundreds of relatives of the missing.
Flight MH370 dropped out of sight an hour after taking off from Kuala Lumpur to Beijing early on Saturday, under clear night skies and with no suspicion of any mechanical problems.
Dozens of planes and ships have already searched tens of thousands of square miles of Malaysia and off both its coasts without finding a trace of the Boeing 777.
Adding to the frustration and uncertainty, Malaysia's military has said the plane could have turned around from its planned flight path, but there were conflicting statements and reports about how far and in which direction it could have flown after communication was lost.
Malaysia's air force chief, Rodzali Daud, denied saying military radar had tracked MH370 flying over the Strait of Malacca off the country's west coast, about 500 km (310 miles) from the point ,roughly midway between the east coast town of Kota Bharu and Vietnam, where it was last seen by air traffic control.
Malaysia's Berita Harian newspaper on Tuesday quoted Rodzali as saying the plane was last detected at the northern end of the Strait of Malacca at 2.40 a.m. on Saturday, more than an hour after it lost contact.
"It would not be appropriate for the Royal Malaysian Air Force to issue any official conclusions as to the aircraft's flight path until a high amount of certainty and verification is achieved," Rodzali said in a statement on Wednesday.
"However all ongoing search operations are at the moment being conducted to cover all possible areas where the aircraft could have gone down in order to ensure no possibility is overlooked."
Indonesia and Thailand, which lie on either side of the northern part of the Malacca Strait, have said their militaries detected no sign of any unusual aircraft in their airspace.
The massive search operation involving ships and aircraft from 10 countries is spread out over the Gulf of Thailand and the South China Sea, which lie between Malaysia and Vietnam, and in the Strait of Malacca extending into the Andaman Sea.
An Indian foreign ministry official said Malaysia has sought its help in the search. India has a large military command in its Andaman and Nicobar islands and its navy patrols in the Malacca Strait.
ONE OF SEVERAL THEORIES
A senior military officer who had been briefed on the investigation told Reuters on Tuesday that the missing aircraft had made a detour to the west after communications with civilian authorities ended.
"It changed course after Kota Bharu and took a lower altitude. It made it into the Malacca Strait," the officer said.
After the comments from the officer, a non-military source familiar with the investigations said the reported detour was one of several theories and was being checked.
If the plane had made such a detour it would undermine the theory that it suffered a sudden, catastrophic mechanical failure, as it would mean it flew at least 500 km (350 miles) after its last contact with air traffic control.
In the absence of any concrete evidence to explain the plane's disappearance, authorities have not ruled out anything. 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.
The airline said it was taking seriously a report by a South African woman who said the co-pilot of the missing plane had invited her and a female companion to sit in the cockpit during a flight two years ago, in an apparent breach of security.
"Malaysia Airlines has become aware of the allegations being made against First Officer Fariq Ab Hamid which we take very seriously. We are shocked by these allegations. We have not been able to confirm the validity of the pictures and videos of the alleged incident," the airline said in a statement.
The woman, Jonti Roos, told Reuters that she and her friend were invited to fly in the cockpit by Fariq and the pilot between Phuket, Thailand, and Kuala Lumpur in December 2011.
"I thought that they were highly skilled and highly competent and since they were doing it that it was allowed," Roos said. "I want to make it clear, at no point did I feel we were in danger or that they were acting irresponsibly."
Hugh Dunleavy, the commercial director of Malaysia Airlines, told Reuters there was no reason to blame the crew.
"We have no reason to believe that there was anything, any actions, internally by the crew that caused the disappearance of this aircraft," he said.
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 on landing in San Francisco, killing three people.
U.S. planemaker Boeing has declined to comment beyond a brief statement saying it was monitoring the situation.