Joint Agency Co-ordination Centre chief Angus Houston said on Monday no pings had been detected since last Tuesday and it was now time for the next step in the search.
He has also revealed an oil slick had been detected in the search area.
A sample has been taken and will be tested when it arrives ashore.
The Australian Defence Force vessel Ocean Shield will cease searching with its towed pinger locator and deploy the autonomous underwater vehicle Bluefin 21 as soon as possible to search the sea floor using its side-scan sonar.
Each mission will take a minimum of 24 hours to complete, including the downloading of data.
In mission number one – to commence on Monday evening – it will cover an area of 40 square kilometres.
He says the sub cannot go any deeper than 4500 metres and it is important to be realistic about its prospects.
“It may be very difficult to find something,” he said.
Air Chief Marshal Houston said it would be “a number of days” until the two litres of oil collected could be conclusively tested.
“I stress the source of the oil is yet to be determined,” he said.
Air Chief Marshal Houston said the four signals, believed to have originated from the aircraft’s black box flight data recorders, still constituted the most promising lead.
Analysis of these signals had allowed determination of a reduced and manageable search area.
Up to 11 military aircraft, one civil aircraft and 15 ships are undertaking Monday’s search for the Boeing 777, which vanished en route from Kuala Lumpur to Beijing on March 8 with 239 people on board.
The search area, about 2200 kilometres northwest of Perth, was narrowed after two sets of “pings”, believed to be from the plane’s flight data and cockpit voice recorders, were detected by Australian Defence vessel Ocean Shield on April 5 and April 8.
There was another acoustic detection on April 10, but search co-ordinators said the following day that it was not believed to be related to the missing passenger jet.
The Ocean Shield will continue to sweep a towed pinger locator near the ocean floor in a bid to pick up more signals and further narrow the search area.
But the chances of that are becoming increasingly unlikely, given the battery on the black box beacons have a 30-day shelf life, although they could last for several days longer.
Houston recently told reporters that he wasn’t “going to confirm anything until someone lays eyes on the wreckage”.
South-easterly winds and possible showers are expected on Monday with sea swells up to 1.5 metres and visibility up to five kilometres.