* Air force chief says unidentified plane tracked 200 miles northwest of Penang
* Still no sign of missing plane, search in fifth day
* 227 passengers and 12 crew aboard lost flight
* Area being searched the size of Hungary
By Eveline Danubrata and Nguyen Phuong Linh
KUALA LUMPUR/PHU QUOC, Vietnam, March 12 (Reuters) - M alaysia's military has traced what could have been the jetliner missing for almost five days to an area south of the Thai holiday island of Phuket, hundreds of miles from its last known position, the country's air force chief said on Wednesday.
After a series of at times conflicting statements, the latest revelation underlined that authorities remain uncertain even where to look for the plane, and no closer to explaining what happened to Malaysia Airlines Flight MH370 or the 239 people on board.
The flight disappeared from civilian radar screens shortly before 1:30 a.m. on Saturday, less than an hour after taking off from Kuala Lumpur, as it flew northeast across the mouth of the Gulf of Thailand bound for Beijing. What happened next is one of the most baffling mysteries in modern aviation history.
Malaysian air force chief Rodzali Daud told a news conference 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.
It was not confirmed that the unidentified plane was Flight MH370, but Malaysia was sharing the data with international civilian and military authorities, Rodzali said.
"We are corroborating this," he added. "We are still working with the experts, it's an unidentified plot."
According to the data from Rodzali, if it was the missing plane it would have flown for 45 minutes and lost only about 5,000 feet (1,500 metres) in altitude.
There was no word on which direction it was headed and still no clue what happened aboard, prolonging the agonising wait for news for hundreds of relatives of those on board.
A position 200 miles northwest of Penang, in the northern part of the Strait of Malacca, would put the plane 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.
The position is hundreds of miles west of the point where the Boeing 777-200ER dropped off air traffic control screens. Malaysia has asked India for help in tracing the aircraft and New Delhi's coastguard planes have joined the search.
Authorities however are continuing to search around both locations - at the last known position of the plane over the Gulf of Thailand and around the radar plotting site where the Malacca Strait meets the Andaman Sea.
In total, the search is over 27,000 square nautical miles (93,000 sq km), an area the size of Hungary or Indiana.
Until now, there has been no confirmed sighting of the plane or any debris.
A dozen countries are helping Malaysia in the search, with 42 ships and 39 aircraft involved, Transport Minister Hishammuddin Hussein said.
"My heart reaches out to the families of the passengers and crew," he said. "And I give you my assurance we will not reduce the tempo and that we will not spare any effort to find the missing plane."
Malaysia has been criticised for giving conflicting and confusing information on the last known location of aircraft.
Earlier on Wednesday, air force chief Rodzali had denied saying military radar had tracked MH370 flying over the Strait of Malacca.
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.
"As long as the plane is not found, we would continue doing our mission," Vo Van Tuan, spokesman for Vietnam Search and Rescue Committee, told reporters in Hanoi.
"We should always keep up hope, there can be miracles, humans can survive for a long time in difficult conditions. We must not give up hope that the missing people are still alive."
NOTHING RULED OUT
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 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.