Signals detected by teams scouring the Indian Ocean for wreckage from flight MH370 have to be from a plane, an expert has told Sky News.
Chris Bellamy, Professor of Maritime Security at the University of Greenwich, has said it is "very difficult" to think what else the pings picked up by a Chinese ship could be.
"That is the frequency of an aircraft black box. Ships have black boxes, but that’s at 400kHz-15kHz, so quite a different range of frequencies. It's very difficult to think what else this could be," he said.
The vessel Haixun 01 picked up two "acoustic events" around 1,000 miles (1,600km) northwest of Perth.
The two pings detected by the Chinese search team had a frequency of 37.5kHz, which Professor Bellamy said had been deliberately chosen for black boxes to stand out.
"The ping has to be some sort of deliberate black box or some artificial source of energy," he said.
"I don't think it could be a natural phenomenon, but it could well be an echo," he added.
Australia's HMAS Ocean Shield has also reported a separate "acoustic event" some 345 miles (555km) away.
But marine salvage expert Captain John Noble said the pings detected by the Australian vessel were not likely linked to the Chinese detection.
"It's almost certain it cannot be the same source. They will be testing the reliability of the signals from the Australian ship and the Chinese ship," he said.
Despite what could be the breakthrough in the search for the Malaysia Airlines jet, Professor Bellamy stressed pinpointing the black box and wreckage from the Boeing 777 could still take some time as the ocean floor is "mountainous".
"The black box could be in a ravine or something like that which might cause refraction of the sound," he said.
If the pings detected are an echo, the source could be located further away then what they think.
"Things rebound, echoes happen under the water as they do in the air," Professor Bellamy said.
Progress is likely to be slow, as vessels must move slow to improve on these initial readings.
Australian Commodore Peter Leavy said: "The search using sub-surface equipment needs to be methodical and carefully executed in order to effectively detect the faint signal of the pinger."
Captain Noble sounded an optimistic note.
Discussing the basic nature of the equipment being used by the Chinese, he said: "If there is a signal and the microphone is out of interference it's actually quite encouraging.
"If that sort of equipment is picking up the signal then the more sophisticated equipment that's on its way can help with the triangulation process."