* Pulse signal picked up in southern Indian Ocean -Xinhua
* Up to 13 planes, 11 ships scouring ocean on Saturday
* Battery of plane's black box likely to run out soon
* Malaysia starts investigation into disappearance of plane (Edits, adds Australian agency comments, other details)
By Siva Govindasamy and Swati Pandey
KUALA LUMPUR/PERTH, Australia, April 5 (Reuters) - A Chinese patrol ship hunting for a missing Malaysia Airlines jetliner detected a pulse signal in the south Indian Ocean on Saturday, the state news agency Xinhua reported, in a possible indicator of the underwater beacon from a plane's "black box".
Australian search authorities said such a signal would be "consistent" with a black box, but both they and Xinhua stressed there was no conclusive evidence linking the "ping" to Flight MH370, which went missing on March 8 with 239 people aboard shortly after taking off from Kuala Lumpur for Beijing.
A black box detector deployed by the vessel Haixun 01 picked up the "ping" signal with a frequency of 37.5kHz per second - the same as emitted by flight recorders - at about 25 degrees south latitude and 101 degrees east longitude, Xinhua said.
Xinhua also reported that a Chinese air force plane had spotted a number of white floating objects in the search area.
Dozens of ships and planes from 26 countries are racing to find the black box recorders before their batteries run out.
Up to 10 military planes, three civilian jets and 11 ships are scouring a 217,000-sq-km (88,000-sq-mile) patch of desolate ocean some 1,700 km (1,060 miles) northwest of Perth, Australia, near where investigators believe the Boeing went down.
"The characteristics reported (by the Chinese vessel) are consistent with the aircraft black box," Retired Air Chief Marshal Angus Houston, head of the Australian agency coordinating the operation, said in a statement.
"However, there is no confirmation at this stage that the signals and the objects are related to the missing aircraft," he said, adding his agency was seeking more information from China.
Authorities have not ruled out mechanical problems as a cause of the plane's disappearance, but say the evidence, including loss of communications, suggests it was deliberately diverted thousands of kilometres (miles) from its set route.
Sonar equipment on two ships joining the search may help find the black box voice and data recorders that are key to unlocking what happened on the flight.
Australian authorities said the so-called Towed Pinger Locator would be pulled behind navy ship HMAS Ocean Shield, searching a converging course on a 240-km (150-mile) track with British hydrographic survey ship HMS Echo.
Experts say the Towed Pinger Locator may be of little use unless investigators can get a much better idea of exactly where the plane went into the water because its limited range and the slow speed at which it must be pulled behind the ship mean it cannot cover large areas of ocean quickly.
"I won't even call it an area. What we are doing is we are tracking down the best estimate of the course that the aircraft was on," U.S. Navy Captain Mark Matthews told Reuters. "It takes a couple of days on each leg so its a slow-going search."
Britain is also sending HMS Tireless, a nuclear submarine with sonar capabilities, and a Malaysian frigate was due to arrive in the search area on Saturday.
"If we haven't found anything in six weeks we will continue because there are a lot of things in the aircraft that will float," Australia's Houston told reporters.
Dozens of flights by a multinational taskforce have failed to turn up any trace of the plane in the past four weeks.
The Boeing 777 was briefly picked up on military radar on the other side of Malaysia and analysis of subsequent hourly electronic "handshakes" exchanged with a satellite led investigators to conclude the plane had crashed far off the west Australian coast hours later.
Malaysian authorities have faced heavy criticism, particularly from China, for mismanaging the search and holding back information. Most of the 227 passengers were Chinese.
Malaysia said on Saturday it had launched a formal investigation into the plane's disappearance that would include experts from Australia, the United States, China, Britain and France.
Normally, a formal air safety investigation is not launched until wreckage is found. But there have been concerns that Malaysia's informal investigations to date have lacked the legal standing of an official inquiry convened under U.N. rules.
Under International Civil Aviation Organisation rules, the country where the aircraft is registered leads the investigation when the incident takes place in international waters.
Malaysia's Defence Minister Hishammuddin Hussein said the investigation would comprise three groups: one would examine maintenance records, structures and systems; an "operations" group would study flight recorders, operations and meteorology; and a "medical and human factors" group would look into psychology, pathology and survival. (Additional reporting by Tim Hepher in Paris, Niluksi Koswanage in KUALA LUMPUR, Jane Wardell in SYDNEY; Writing by Siva Govindasamy and Mark Bendeich; Editing by Nick Macfie and Gareth Jones)