Chinese and Australian ships hunting for a missing Malaysia Airlines plane have picked up separate acoustic signals in different parts of a vast Indian Ocean search area.
They are trying to verify if one could be from the plane's black box recorders.
Australian search authorities said a Chinese patrol vessel, the Haixun 01, had picked up a fleeting "ping" signal twice in recent days in waters west of Perth.
The signal was detected near where investigators believe Malaysia Airlines Flight MH370 went down on 8 March with 239 people on board.
More planes and ships are being sent to assist in that area.
Meanwhile, Australia's HMAS Ocean Shield reported a separate "acoustic event" about 300 nautical miles away.
The Ocean Shield is carrying sophisticated US Navy equipment designed to pick up signals sent from the black boxes.
The voice and data recorders may hold the key to explain why the aircraft ended up thousands of kilometres off course.
"We are treating each of them seriously. We need to ensure before we leave any of those areas that this does not have any connection with MH370," said retired Air Chief Marshal Angus Houston, the head of the Australian agency coordinating the operation.
A black box detector deployed by the Haixun 01 picked up the signal with a frequency of 37.5kHz per second - the same as emitted by flight recorders - at about 25 degrees south and 101 degrees east.
Australian search authorities said such a signal would be consistent with a black box, but both they and the Chinese stressed there was no conclusive evidence linking it to the Boeing 777.
Mr Houston said analysis of earlier satellite data had again led investigators to refine the search area towards the southern part of the corridor.
He said: "The area of the highest probability is, what we think, the southern part where Haixun 01 is operating. That is why we are really interested in the two acoustic encounters that Haixun 01 has had."
The water is around 4,500 metres (14,800ft) deep in that part of the search area.
Australian Prime Minister Tony Abbott said he was "hopeful but by no means certain" that the reported pulse signals were related to MH370.
"This is the most difficult search in human history. We are searching for an aircraft which is at the bottom of a very deep ocean and it is a very, very wide search area," Mr Abbott told reporters in Tokyo, where he is on a visit.
Up to a dozen planes and 13 ships will be searching three separate areas about 1,240 miles northwest of Perth, Australia's Joint Agency Coordination Centre said.