By Morag MacKinnon
HMAS STIRLING NAVAL BASE, Australia (Reuters) - An Australian naval vessel carrying an underwater drone involved in the search for Malaysia Airlines Flight MH370 left port on Saturday on its second mission to scan part of the Indian Ocean where the longest sonar "ping" was heard over a month ago.
The Ocean Shield is heading to the area where a signal was first located and heard for some two hours on April 5, about 1,600 km (1,000 miles) northwest of Perth to launch the Bluefin-21 submersible.
More than two dozen countries have been involved in the hunt for the Boeing 777 that disappeared from radar shortly after taking off from Kuala Lumpur for Beijing with 239 people, mostly Chinese, on board in one of the world's greatest aviation mysteries.
Weeks of daily sorties have failed to turn up any trace of the plane, even after narrowing the search to an arc in the southern Indian Ocean. Batteries on the black box voice and data recorders have gone flat.
The search had been centered on a 314-square-km (121-sq-mile) area around the second "ping" located and monitored for about 13 minutes on April 5, and which search authorities identified as their strongest lead.
With the search of that area complete, the focus is shifting to the area where the first, and longer, signal was detected the same day, U.S. Navy Captain Mark Matthews told journalists at a naval base near Perth.
"What you do is you go look at your best indications and you pursue them until they're exhausted," he said. "These things don't happen fast. These searches don't happen on the hours or days cycle. These happen on the weeks and months cycle."
Australia, China and Malaysia earlier this week pledged not to give up searching for the plane, even though air and surface searches for debris have been abandoned.
The Ocean Shield returned to Stirling Naval Base south of Perth earlier this week after more than a month at sea to resupply, change crew and perform software modifications and maintenance on the Bluefin.
The submersible has dived to a maximum depth of 5,005 meters in its daily 20-hour missions to scan the ocean floor using sonar, despite being only designed to dive to 4,500 meters, Matthews said.
With just three weeks left on loan from the U.S. Navy, the pressure is on about how to proceed and who will pay for the next phase of the search. The Ocean Shield, which will take three days to arrive at the search location, is due back in port by the end of the month.
Last week, Malaysia released its most comprehensive account yet of what happened to Flight MH370, detailing the route the plane probably took as it veered off course and the confusion that followed.
The officials have said the focus will be on 60,000 sq km (23,000 sq mile) of seabed in the Indian Ocean that could take a year to search.
(Editing by Nick Macfie)