The Bluefin 21 unmanned sub began its eighth dive off the west coast of Australia yesterday morning.
It is searching a 6.2-mile diameter circle centred on an underwater signal believed to have come from the aircraft's black boxes before their batteries died.
The sonar scan of the seabed in that area is expected to be completed in five to seven days, the search centre said.
Malaysia's defence minister Hishammuddin Hussein said: "The narrowing of the search … is at a very critical juncture, so I appeal to everybody around the world to pray and pray hard that we find something to work on over the next couple of days." He said there were no plans to give up once the Bluefin finishes work. The scope of the search may be broadened or other assets may be used.
"The search will always continue," he said. "It is just a matter of approach. All efforts will be intensified for the next few days."
Meanwhile 11 aircraft and 12 ships continued to scan the ocean surface for debris from the Boeing 777 that vanished on March 8 en route from Kuala Lumpur to Beijing with 239 people on board.
Radar and satellite data show it mysteriously veered far off course and would have run out of fuel in the remote area of the southern Indian Ocean where the search is focused. No debris has been recovered yet.
The search has tormented the families whose loved ones were aboard Flight 370. About two-thirds of the passengers were Chinese.
The most promising development came when four underwater signals were detected on April 5 and 8.
The sounds were consistent with pings that would have been emanating from the flight-data and cockpit recorders' beacons before their batteries died.
The underwater operation is complicated by the depth of the largely unexplored silt-covered sea floor. The US Navy's submarine has gone beyond its recommended limit of 4500 metres.