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 12:28 (local time, 1:28 am Malaysian time), six minutes after MH370 vanished" in the South China Sea moving southwest back towards its origin in the Malaysian capital Kuala Lumpur and the Strait of Malacca, he told AFP.
That timing corresponds with the last transmission from the aircraft's transponder at 1:21 am Malaysian time, which relayed information about the plane's altitude and location.
Although the signal was sporadic, the aircraft was later again picked up by Thai radar swinging north and disappearing over the Andaman Sea, Monthon added.
"It's not confirmed that the aircraft is MH370," he said, without giving the exact times of the later sightings.
The revelation is 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.