Ships listening for undersea signals from Flight MH370 have failed to detect any more "pings" and will spend several more days trying to pinpoint a crash site before a mini-sub is launched to scour the seabed, searchers said Tuesday.

Exactly one month since the Boeing 777 vanished with 239 people on board, optimism over the earlier signals in the Indian Ocean dimmed, with time running out to detect further signals as the batteries in beacons on the jet's black box data recorders reach their expiry date.

"We need to continue (listening) for several days to the point at which there is absolutely no doubt that the pinger batteries will have expired," said search chief Angus Houston.

A day earlier, Houston had labelled "very encouraging" the signals picked up by a specialised US Navy device towed deep underwater by the Australian vessel Ocean Shield.

If the transmissions can be recaptured and confirmed as emanating from the Malaysia Airlines plane's data recorders, technicians are poised to deploy a submersible drone, the Bluefin-21, far off western Australia.

The US-made device uses sonar to scan the seafloor for possible crash debris.

The plane mysteriously vanished March 8, diverting en route from Kuala Lumpur to Beijing for reasons unknown. The search is focused on a 600-kilometre (370-mile) arc of the remote southern Indian Ocean.

The Ocean Shield is criss-crossing an area where satellite data indicates the plane may have gone down, trying to home in on the signals.

- 'Many, many, many days' -

"If we go down there now and do the visual search it will take many, many, many days because it's very slow, very painstaking work to scour the ocean floor," Houston said.

If the Bluefin-21 detects something, it can resurface and be sent back down with a video camera to seek visual evidence of a crash.

Families of MH370 passengers in Beijing marked the one-month anniversary with a tearful candlelit vigil Tuesday, still in agonising suspense as they await evidence of the fate of their loved ones.

"We've been waiting and holding on here for already 31 days," said Steve Wang, one of the relatives.

About two-thirds of the 239 people on board were Chinese.

Selamat Omar, a Malaysian whose 29-year-old son Khairul was on board, held on to improbable hopes that he might still be alive.

"Until they can find the black box or the wreckage I will hold on to the belief that the passengers are safe," he said.

"I will only accept that there are no survivors when we find the debris of the plane in the sea."

- Taking it to the limit -

But a daunting task lies ahead, especially if the pings cannot be reacquired to narrow the search area.

The 4.5-kilometre (nearly three-mile) depth of the ocean floor is the absolute operating limit for a Bluefin-21, which is designed for deep-sea surveying.

Ocean Shield, which earlier picked up two series of pulses, one lasting two hours and 20 minutes and the other 13 minutes, is operating at the northern end of the defined search area.

The Chinese ship Haixun 01 and Britain's HMS Echo are working the southern end.

Gerry Soejatman, an independent aviation analyst based in Jakarta, said if a crash site cannot be pinpointed, underwater sonar mapping can take an entire day for an area the size of a football field, and possibly "a month, or maybe more," for a one-square-kilometre section.

"It is crucial to find the location where the signal is coming from. Time is running out. If the signal stops, it will be a much harder task," he said.

Commander William Marks of the US Seventh Fleet had said earlier one of the signals strengthened for a time and then weakened as the Ocean Shield swept by.

He said this indicated crews were near its source as a signal strengthens when approached, and weakens when left behind.

Around a dozen planes also continued scanning the area 2,200 kilometres northwest of Perth, looking for floating debris.

burs-dma/ac