The last communication received from a Malaysia Airlines plane suggests everything was normal on board minutes before it went missing over the South China Sea, Malaysian authorities say.
Flight MH370 replied "All right, roger that" to a radio message from Malaysian air control, authorities said.
The search has been widened to waters off both sides of the peninsula.
Malaysia's air force chief has denied reports the plane was tracked to the Malacca Strait in the west.
The China-bound plane went missing on Saturday with 239 people on board.
It vanished 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.
Malaysian authorities revealed the plane's last communication at a news conference held in Beijing for relatives of the 154 Chinese who are among the missing passengers.
As the plane reached the boundary between Malaysian and Vietnamese airspace, the Malaysian air control announced it was handing over to Ho Chi Minh City Control.
Minutes later, all contact with Flight MH370 was lost.
China's foreign ministry said conflicting information about the plane's flight path was "pretty chaotic".
Earlier on Wednesday, Malaysia's air force chief Rodzali Daud denied remarks attributed to him in local media that flight 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".
Early search efforts focused 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.
Operations are now covering some 27,000 square nautical miles (92,000 sq km).
Malaysian authorities on Wednesday requested assistance from India in searching the Andaman Sea, north of the Malacca Strait.
Vietnam has confirmed an investigation into a possible sighting of the plane has so far yielded no results.
Vietnam's air traffic management earlier said it had received an email from a New Zealander working in 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," deputy general director Doan Huu Gia said.
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'
Some 42 ships and 39 aircraft from several 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.
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".
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".