Teams searching for a missing Malaysian plane say they will wait for further contact with signals picked up at the weekend before sending a submersible down to search for debris.
Australian Navy ship the Ocean Shield has heard signals that officials say could be consistent with "black box" flight recorders, but has not been able to reacquire them since Sunday.
The Malaysia Airlines plane with 239 passengers and crew vanished on 8 March en route from Kuala Lumpur to Beijing. Investigators believe flight MH370 crashed somewhere in the southern Indian Ocean, west of the Australian city of Perth and thousands of kilometres from its intended flight path. No confirmed debris has yet been found.
In Kuala Lumpur and Beijing, vigils have been held to mark one month since the plane disappeared.
The search entered its 32nd day on Monday, two days beyond the expected life of batteries on the plane's black boxes.
Australian officials say it is key to re-find the signals before using the submersible in a search for wreckage, the BBC reports.
Speaking at Pearce Airbase in Perth, Australian Defence Minister David Johnston said that several days of "intense action" were ahead as search teams tackled "this difficult, complex task" while black box pingers were still believed to be active.
The Ocean Shield has been using a towed pinger locator to listen for transmissions from the plane's flight recorders. Over the weekend, it heard signals on two separate occasions, the first time for about two hours and 20 minutes, and a second time for about 13 minutes.
On the second occasion two distinct pinger returns were audible, something said to be consistent with transmissions from both the flight data recorder and the cockpit voice recorder.
Air Chief Marshall Angus Houston, who heads the agency overseeing the search for the plane, said that Ocean Shield had had no further contact with any transmissions since then.
He said that work involving the towed pinger locator would continue for several days, and the Bluefin 21 underwater drone would not be deployed unless more transmissions were received.
Wing Commander Andy Scott oversees the New Zealand participation in the search and said on Tuesday their Orion has been in use for almost a month now.
"This is certainly a very long search that we're engaged in. From New Zealand's point of view, the search area that's being looked at each day is roughly the same size as the country of New Zealand, but we've got a very committed team there, they're very dedicated to the task and we're looking forward to getting back and flying again tomorrow."
The search is focusing on a 600km arc in the southern Indian Ocean, about 1600km off the West Australian coast.