Hopes were raised last night that the missing Air Malaysia flight MH370 may have been found after a Chinese naval ship searching the Indian Ocean for trace of the passenger jet had picked up intermittent ultrasonic signals. Chinese state media reported that pulses at the right frequency for a plane's black box recorder had been detected for 90 seconds yesterday and for 15 minutes on Friday, but stressed they had not been linked conclusively to the airliner. White debris was also seen on the surface of the sea about 90km away.

The news was relayed by China to the Malaysian government, it was reported. Australia's Joint Agency Coordination Centre (JACC), which is running the search operation, said it had been told the signals were "consistent with the aircraft black box".

However, it could not say whether or not they came from the plane, and some relatives of those on board were sceptical. Jack Song, whose sister was on the plane, told CNN: "I don't believe it. There's no piece of debris, so how can you find the black box? So you can just wait for more news tomorrow. Maybe it will be another mistake."

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Jiang Hui, another passenger's relative, said in a text message: "There is no confirmation, and we are all waiting patiently."

But several experts said it was possible or even probable that the signal was coming from the two black boxes. Their batteries last for about a month, so the signals are due to run out around today.

The disappearance on 8 March of the plane, which was carrying 239 people, remains a mystery. Authorities have not ruled out mechanical problems, but say the evidence suggests it was deliberately diverted and crashed in the Indian Ocean.

It is hoped that the flight data recorder and cockpit voice recorder will contain enough information to establish what happened. Sound is recorded on microphones on the pilots' and engineer's headsets and another general microphone in the cockpit, although only for an hour or possibly two up to the point of impact. Some flight data recorders even have details about whether or not passenger seatbelts are fastened.

Phil Giles, an air safety investigator who previously worked for the UK Air Accidents Investigation Branch, said the discovery of the signals was promising. "That's not going to be a whale or a porpoise or a squid or anything like that. It's got to be a mechanical device," he said.

"If that [report] is kosher, then it's probably coming from a pinger on a black box. I would think that it's very unlikely that somebody has dumped another black box down there less than a month ago in the Indian Ocean. The chances are pretty good that it's from MH370."

Mr Giles said the signal could be dropping in and out because the batteries have nearly run out or because the plane is lodged in "mountainous" underwater terrain.

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Chris Yates, an aviation expert, also said that the report "sounds positive". "On the face of it, it's good news, but we have to wait and see," he said.

The crew of the Chinese ship which picked up the signals, the Haixun 01, were reportedly concerned that the signal detected on Friday may have been the result of interference from other vessels in the area and had wanted to carry out further tests.

Air chief marshal Angus Houston, the chief co-ordinator of JACC, said in a statement that he had been "advised that a series of sounds have been detected by a Chinese ship in the search area".

"The characteristics reported are consistent with the aircraft black box," he said. "A number of white objects were also sighted on the surface about 90km from the detection area. However, there is no confirmation at this stage that the signals and the objects are related to the missing aircraft."

He stressed that the agency "cannot verify any connection to the missing aircraft", adding that they were asking authorities in China for more information. "The deployment of RAAF [Royal Australian Air Force] assets to the area where the Chinese ship detected the sounds is being considered," he said.

Earlier, Australian Defence Minister David Johnston had urged caution. "I have not had a chance to get to the bottom of this, but can I tell you this is not the first time we've had something that has turned out to be very disappointing. There's a huge chance of false positives here," he said.

Up to 10 military planes, three civil jets and 11 ships were involved in the search effort yesterday, including the UK's specialist detection vessel HMS Echo. The air crash investigation team includes analysts from Malaysia, the US, Britain, China and Australia. Asked about the cost of running such as large operation, Hishammuddin Hussein, Malaysia's defence minister, said financial concerns were immaterial. "Malaysia will not stop looking for MH370," he said.