Search crews in the southern Indian Ocean are urgently trying to relocate electronic transmissions they believe came from the missing Malaysian airliner's black box.
No signals have been heard since Sunday, when U.S. Navy equipment detected two distinct sounds consistent with acoustic pings from the plane's flight data and cockpit voice recorders.
Australian search chief Angus Houston said at a news conference Tuesday that without more pings, authorities are unable to deploy a robot to search the ocean floor for wreckage.
"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. And of course the depths are very deep and it's very challenging."
Houston said the search for underwater signals will likely take "several days."
Authorities are running out of time, since the batteries on the black box's locator beacon are built to last for around 30 days. The plane disappeared one month ago Tuesday.
Up to 14 planes and 14 ships face favorable weather conditions Tuesday as they continue the search in a 80,000-square-kilometer area northwest of the Australian city of Perth.
The ocean there has an average depth of about 4.5 kilometers, making it extremely difficult for authorities to find any debris or locate the black box's faint electronic signals.
Officials have said it is increasingly unlikely that any wreckage will be found on the surface of the water because of natural ocean drift and recently stormy seas.
Chinese relatives of the missing passengers gathered in Beijing late Tuesday for a candlelight vigil to mark the one-month anniversary. Steve Wong, a spokesman for the families, said they will not give up hope until wreckage is found.
"It is not the time for memorial ceremonies yet. We need to persist in resolving this situation, to persist in finding the clues of what really happened and finding our relatives. This is the argument that we have been persistently raising since the first day and will continue to do it in the future."
The families are upset at what they feel is a lack of transparency by Malaysian authorities, as well as several false leads in the search for the plane.
The Boeing disappeared early on March 8 with 239 people aboard on a flight from Kuala Lumpur to Beijing.
The plane vanished without any distress calls, and authorities have refused to rule out any possibilities, including hijacking, sabotage or a mechanical malfunction.
Malaysian officials say they are continuing to investigate whether any deliberate action by the pilots or crew could have sent the plane down.