Search teams are scouring waters off both sides of the Malaysian peninsula, amid confusion over a missing Malaysia Airlines plane’s last known location.
Malaysia’s air force chief has denied reports that the plane was tracked to the Malacca Strait in the west.
Vietnam has despatched a plane to investigate an eyewitness report of a possible object burning in the sky east of Vietnam.
Flight MH370 went missing on Saturday. It had 239 people on board.
Authorities have been searching for the plane, which disappeared en route from Kuala Lumpur to Beijing, for the past five days.
Earlier this week, Malaysia widened the search for the missing plane amid conflicting reports on its last known position.
The Malaysian authorities initially said flight MH370 disappeared about an hour after it took off from Kuala Lumpur International Airport, as it flew over the South China Sea, south of Vietnam’s Ca Mau peninsula. No distress signal or message was sent.
Early search efforts focussed on waters between Malaysia and Vietnam.
The search was later extended to the Malacca Strait and the Andaman Sea, off Malaysia’s west coast, amid reports that the plane could have turned back.
On Wednesday, Malaysia’s air force chief Rodzali Daud denied remarks attributed to him in local media that a missing Malaysia Airlines plane was tracked by military radar to the Malacca Strait, far west of its planned route.
Gen Rodzali Daud said he “did not make any such statements”, but the air force had “not ruled out the possibility of an air turn-back”.
On Wednesday, authorities also began searching the Andaman Sea, north of the Malacca Strait.
“We are not going to leave any chance. We have to look at every possibility,” Malaysian civil aviation chief Azharuddin Abdul Rahman told AFP news agency, without indicating why the search was expanded north.
Malaysia had requested assistance from India in searching areas near the Andaman Sea, India’s foreign ministry said.
Meanwhile, Vietnam said it had deployed aircraft to investigate a possible sighting of the plane.
Doan Huu Gia, deputy general director of Vietnam’s air traffic management, said: “We received an email from a New Zealander who works on one of the oil rigs off Vung Tau.
“He said he spotted a burning [object] at that location, some 300 km (200 miles) southeast of Vung Tau.”
Officials still do not know what went wrong with the aircraft, and several leads pursued so far have proven not to be linked to the plane.
After more than four days of fruitless searching, there is an element of desperation creeping into this operation, the BBC’s Jonathan Head in Kuala Lumpur reports.
‘Shock at allegations’
At least 40 ships and 34 aircraft from several different countries are taking part in the search for the plane.
Two-thirds of the passengers on board the plane were Chinese. Some were from a range of other Asian countries, North America or Europe.
On Tuesday it emerged that two men travelling on stolen passports on board the plane were Iranians with no apparent links to terrorist groups, officials said.
Meanwhile, Malaysia Airlines said in a statement that it was “shocked” by reports made against its First Officer, Fariq Ab Hamid, who was the co-pilot of the missing flight.
A South African tourist told Australia’s Channel Nine that she and her friend were invited to sit in the cockpit with Fariq Ab Hamid and the pilot during a flight in 2011, in an apparent breach of airline rules.
Malaysia Airlines said it took the reports “very seriously”.
“We have not been able to confirm the validity of the pictures and videos of the alleged incident. As you are aware, we are in the midst of a crisis, and we do not want our attention to be diverted,” it said.
None of the debris and oil slicks spotted in the South China Sea or Malacca Strait so far have proved to be linked to the disappearance.
In the US, CIA Director John Brennan said the possibility of a terror link could not be ruled out. But he said “no claims of responsibility” over the missing jet had “been confirmed or corroborated”.