Specialists prepare to deploy the towed pinger locator on the Australian vessel Ocean Shield.

Specialists prepare to deploy the towed pinger locator on the Australian vessel Ocean Shield. Photo: Reuters

A series of pings detected in the southern Indian Ocean and originally believed to have come from missing Malaysia Airlines jet MH370 are now thought to have been emitted from either the searching ship itself or equipment used to detect the pings, a US Navy official says.

Michael Dean, the US Navy's director of ocean engineering, told CNN that authorities now believed the four acoustic pings at the centre of the search off the West Australian coast did not come from the missing passenger jet's black boxes, but from a "man-made source".

"Our best theory at this point is that (the pings were) likely some sound produced by the ship ... or within the electronics of the Towed Pinger Locator," Mr Dean told CNN on Wednesday.

"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."

He said other countries involved in the massive search for the jet, which disappeared on March 8 with 239 people on board, had also reached the same conclusion.

When the pings were first detected in early April, retired air chief marshal Angus Houston, the head of the search's Joint Agency Co-ordination Centre (JACC), said experts believed the signals were consistent with those of a flight data recorder.

He said the first two pings - detected on April 5 at 4.45pm and at 9.27pm Perth time - had been analysed by the Australasian Joint Acoustic Analysis Centre, based at HMAS Albatross in Nowra, on the NSW south coast.

"The analysis determined that a very stable, distinct and clear signal was detected at 33.331 kilohertz, and that it consistently pulsed at a 1.106-second interval," Mr Houston said at the time.

''They therefore assess that the transmission was not of natural origin, and was likely sourced from specific electronic equipment. They believe the signals to be consistent with the specification and description of a flight data recorder.''

The final two pings were detected on April 8 - at 4.27pm and 10.17pm, Perth time.

But despite an extensive underwater search, no evidence of the plane has been found in the search area in the southern Indian Ocean.

Fairfax Media has contacted JACC for comment.

Chris Johnson, a spokesman for the US Navy, described Mr Dean's comments as "speculative" and "premature".

He said the US Navy would "defer to the Australians, as the lead in the search effort, to make additional information known at the appropriate time".

"Regarding the news that was reported earlier, Mike Dean's comments today were speculative and premature, as we continue to work with our partners to more thoroughly understand the data acquired by the Towed Pinger Locator," he said in a statement.

With Anne Davies