Thai radar picked up an "unknown aircraft" minutes after flight MH370 last transmitted its location but officials failed to report the findings earlier as the plane was not considered a threat, the air force said Wednesday.
The information emerged during checks of radar logs on Monday -- nine days after the Malaysia Airlines jet carrying 239 passengers and crew disappeared -- after a request from the Malaysian government, according to Air Marshal Monthon Suchookorn.
An "unknown aircraft was detected at 00:28 (local time, 1:28 am Malaysian time), six minutes after MH370 vanished" in the South China Sea, moving southwest towards Kuala Lumpur and the Strait of Malacca, he told AFP.
Planning could hold key to disappearance of jet
Malaysia plane's flight path changed by cockpit computer: report
Why do airplane transponders have an 'off switch?'
That timing corresponds with the last transmission from the Boeing 777's transponder at 1:21 am Malaysian time, which relayed information about the plane's altitude and location.
The timing of the plane being spotted travelling in the opposite direction from MH370's intended flight path to Beijing also comes after the final voice communication from the jet, a seemingly relaxed "All right, good night" at 1:19 am.
Malaysia Airlines believes it was the co-pilot speaking from the cockpit.
Monthon said that although the signal was sporadic, the aircraft was later again picked up by Thai radar swinging north and disappearing over the Andaman Sea.
"It's not confirmed that the aircraft is MH370," he said, adding he was unable to give "exact times" of the later sightings.
The plane slipped off Malaysian civilian radar screens at 1:30 am but continued to blip on its military radars until 2:15 am before disappearing entirely.
The Thai revelations are likely to fuel anger at the apparently sluggish and at times contradictory official response to the jet's disappearance, which has left anguished relatives pleading for answers on the fate of their loved ones.
The Thai air force did not check its records because the aircraft was not in "Thai airspace and it was not a threat to Thailand", the spokesman said, denying it had been "withholding information".
Initially the massive search for the vanished jet focused on the Gulf of Thailand and adjacent South China Sea, with several nations sending boats, helicopters and jets to scour the waters.
The investigation into the fate of the Boeing 777 has focused on findings it was likely deliberately diverted from its flight path to Beijing, probably by someone in the cockpit with advanced aviation skills.
But the drip-feed of often conflicting information from Malaysia has sparked fury among desperate relatives and condemnation from Chinese authorities. Two-thirds of those on board were Chinese.
Twenty-six countries are now involved in the hunt which covers a vast arc of land and sea, in a northern corridor over south and central Asia, and a southern corridor stretching deep into the southern Indian Ocean towards Australia.