"Authorities now almost universally believe the pings did not come from the onboard data or cockpit voice recorders but instead came from some other man-made source unrelated to the jetliner that disappeared on March 8," US Navy deputy director of ocean engineering Michael Dean was quoted as saying by CNN.
"If the pings had come from the recorders, searchers would have found them," he said.
His remarks came as searchers wrapped up the first phase of the search, having scanned 329 square miles of southern Indian Ocean floor without finding any wreckage from the jet.
Autonomous underwater vehicle Bluefin 21, a US Navy probe equipped with side-scan sonar, was deployed last month from the Australian navy ship Ocean Shield to map the ocean floor.
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The Beijing-bound Malaysia Airlines flight MH370 - carrying 239 people, including five Indians, an Indo-Canadian and 154 Chinese nationals - had mysteriously vanished on March 8 after taking off from Kuala Lumpur.
Officials had believed the four signals detected between April 5 and 8 in the area could be from the plane's black box, which contain flight data and cockpit voice recordings.
"Always your fear any time you put electronic equipment in the water is that if any water gets in and grounds or shorts something out, that you could start producing sound," Dean explained.
He said it is not possible to absolutely exclude that the pings came from the black boxes, but there is no evidence now to suggest they did.
"Dean's comments ... Were speculative and premature, as we continue to work with our partners to more thoroughly understand the data acquired by the Towed Pinger Locator," Navy spokesman Chris Johnson said in a statement.
The mystery of the missing plane continued to baffle aviation and security authorities who have so far not succeeded in tracking the aircraft despite deploying hi-tech radar and other gadgets.
Angus Houston, the head of Perth-based Joint Agency Coordination Centre which has been overseeing the search, last week ordered a "robust" review of all of the data about the missing plane, amid claims that Boeing 777 jet might not be in the southern Indian Ocean.