Helicopters were scrambled on Monday to a floating "yellow object" that rescue teams believed could be a life raft from the missing Malaysian Airlines plane - but it turned out to be a false alarm.
Pham Quy Tieu, Vietnam's vice transport minister and deputy head of the its rescue committee, said that a search plane had initially been unable to get close enough to determine what it was.
However, officials later said it was a "moss-covered cap of a cable reel."
The Civil Aviation Authority of Vietnam said on its website searches were being conducted about 90 miles southwest of Tho Chu island, which is located about 120 miles off the coast of southern Vietnam.
On Sunday, NBC News reported that no electronic "chatter" has been detected indicating any known terror group was behind the Boeing 777's mysterious disappearance.
Two passengers on board Malaysian Airlines flight were traveling on stolen passports. Both ticket holders were due to connect to a flight to Amsterdam. One of the passengers was then scheduled to head to Frankfurt with the other en route to Stockholm.
The red-eye flight was carrying 229 people when it vanished en route from Kuala Lumpur, Malaysia, to Beijing in relatively clear weather. No distress signal was sent.
"This is an unprecedented mystery," Azharuddin Abdul Rahman, director of Malaysia's Department of Civil Aviation, told a news conference on Monday.
He said a hijacking had not been ruled out: "We have not narrowed down any possibilities."