A Chinese patrol ship has detected an ultrasonic pulse close to where the missing Malaysia Airlines plane is believed to have crashed into the southern Indian Ocean, state media announced late on Saturday.

Australian search authorities said such a signal would be "consistent" with a black box but both they and China's state news agency, Xinhua, stressed there was no conclusive evidence linking the "ping" to flight MH370 as the search entered its fifth week. Xinhua, which has a reporter on board the vessel, said the pulse had been detected by the patrol ship Haixun's black box detector at around 25 degrees south and 101 degrees east, within the 84,000-sq-mile search zone.

The pulse had a frequency of 37.5kHz – the same as emitted by flight recorders. Dozens of ships and planes from 26 countries, including the British nuclear submarine HMS Tireless, are racing to find the black box recorders before the batteries on their locator beacons run out. No wreckage has yet been found in the area, despite a massive international hunt.

Xinhua also said that a Chinese air force plane involved in the search had spotted a number of floating objects in the search area but it was not clear whether these were close to where the pulse signal was reportedly detected.

The Beijing-bound Boeing 777 disappeared in the early hours of 8 March, shortly after taking off from Kuala Lumpur, with 239 people on board. Investigators have used analysis of the plane's communication with satellites to identify the search area in the southern Indian Ocean, just over 1,000 miles north-west of Perth.

Xinhua's announcement is the first potentially positive sign in the race against time to find the aircraft's black box. Earlier on Saturday Angus Houston, who heads the Australian centre co-ordinating the international operation, warned that it was "getting pretty close" to the point at which the beacons on the flight data and cockpit voice recorders would stop emitting signals. They are certified to send out pulses for 30 days, although experts say the batteries can often last for another 14 days or so.

Anish Patel, president of Dukane Seacom – which has said it made the beacons for the flight data and cockpit voice recorders on board MH370 – told CNN that the pulse was "identical" to the standard beacon frequency. A reporter for Chinese state television said that the signal was heard for around a minute and a half.

David Gallo, who helped to lead the hunt for Air France flight 447 in the Atlantic in 2009, told the Observer that it was not unusual for the sound to come and go because factors such as thermal currents could affect how far it carried.

The Air France search had a false alarm over a possible signal from the black box, he cautioned. But he added that in this case the pulse was unlikely to occur naturally. "It could very well be one of the beacons," he said.

Gallo, who works at the Woods Hole Oceanographic Institution, said that even if pinger locators could not detect the precise location, they might narrow it to an area of four square miles, at which point teams would map the sea floor using robots and towed systems. He added: "When you find [recorders] there is a sense of satisfaction, but it is also a very sombre moment … It brings the end that the families and loved ones of passengers have been praying and hoping would not come."

Earlier in the day Xinhua announced that Chinese planes had photographed white floating objects in the search zone – but there have already been a string of similar false alarms.

The decision to release the news via Chinese media rather than the Australian agency set up to co-ordinate the operation is likely to cause friction. CNN said an Australian source connected to the search said the centre had learned of the alert several hours earlier but had not been able to communicate directly with the Haixun. While other search crews report possible evidence connected to the flight directly to the joint research centre, the Chinese teams report it to Beijing first.

It also said an Australian Defence Force spokesman described the detection of the pulse as "an anomaly of interest".

More than 150 of those on board the flight were Chinese nationals; China has contributed several ships and vessels to the search.

Earlier Hishammuddin Hussein, Malaysia's defence minister and acting transport minister, told a briefing in Kuala Lumpur that the expense of the search was immaterial compared to the importance of establishing what happened for the relatives of those on board. "I can only speak for Malaysia, and Malaysia will not stop looking for MH370," he said.

The minister announced that an independent investigator would be appointed to oversee three teams pursuing the main lines of inquiry: the plane's airworthiness, including its maintenance, structures and systems; operational issues such as flight recorders and meteorology; and medical and human factors.

Investigators believe MH370 was deliberately diverted from its course but experts say that without the flight data and cockpit voice recorders there is little hope of establishing who was responsible and why. Even then, they caution that the information may not shed much light on the mystery. Cockpit voice recorders hold audio from only the last two hours of the flight and the critical events are likely to have occurred much earlier.