Teams searching for the missing Malaysia Airlines plane have reacquired signals that could be consistent with “black box” flight recorders.
An Australian vessel heard the signals again on Tuesday afternoon and evening, the search chief said.
Signals heard earlier had also been further analysed by experts who concluded they were from “specific electronic equipment”, he said.
Flight MH370 disappeared on 8 March, carrying 239 people.
It was travelling from Kuala Lumpur to Beijing when it lost contact with air traffic controllers.
Malaysian officials say that based on satellite data, they believe it ended its flight in the southern Indian Ocean, thousands of kilometres from its intended flight path.
“I believe we are searching in the right area,” said Air Chief Marshal Angus Houston, who heads the joint agency co-ordinating the search.
“But we need to visually identify aircraft wreckage before we can confirm with certainty that this is the final resting place of MH370.”
The Australian vessel, Ocean Shield, has been towing a US Navy pinger locator to listen for signals from the plane’s flight recorders in waters west of the Australian city of Perth.
It twice acquired signals over the weekend.
On Tuesday, it located the signals again, the first time for five minutes and 32 seconds, and the second time for around seven minutes, said ACM Houston.
“Ocean Shield has now detected four transmissions in the same broad area,” he said. “Yesterday’s signals will assist in better defining a reduced and much more manageable search area on the ocean floor.
The signals have been heard in sea with a depth of 4,500 metres (15,000 feet).
ACM Houston said it was important to refine the search area as much as possible before sending down the Bluefin 21 underwater drone to search for wreckage.
“Now hopefully with lots of transmissions we’ll have a tight, small area and hopefully in a matter of days we will be able to find something on the bottom,” he said.
Experts at the Australian Joint Acoustic Analysis Centre had also analysed the first two signals heard over the weekend, he added.
Their analysis showed that a “stable, distinct and clear signal” was detected. Experts had therefore assessed that it was not of natural origin and was likely to be from specific electronic equipment.
“They believe the signals to be consistent with the specification and description of a flight data recorder,” ACM Houston said.
Search teams have been racing against time to locate signals from the flight recorders before their batteries expire after about one month.
Investigators still do not know why MH370 strayed so far off course, after disappearing over the South China Sea between Malaysia and Vietnam.
The backgrounds of both passengers and crew have been scrutinised as officials considered hijacking, sabotage, pilot action or mechanical failure as possible causes.
So far, there is no official explanation for what might have happened.