* Pulse signal picked up in southern Indian Ocean - Xinhua
* Australian PM cautious on possible breakthrough
* Up to 12 planes, 13 ships search three zones on Sunday
* Malaysia starts formal investigation into disappearance of plane
By Lincoln Feast and Swati Pandey
SYDNEY/PERTH, Australia, April 6 (Reuters) - Australian Prime Minister Tony Abbott said on Sunday searchers were "hopeful but by no means certain" that a pulse signal reportedly detected by a Chinese ship in the Indian Ocean was related to a Malaysia Airlines jetliner missing for four weeks.
Chinese state news agency Xinhua reported that a patrol vessel hunting for Flight MH370 had picked up a "ping" on Saturday, raising hopes that it could be from the underwater beacon of the plane's "black box" voice and data recorders.
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 it to the Boeing 777 that went missing on March 8 with 239 people aboard shortly after taking off from Kuala Lumpur for Beijing.
"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," Abbott told reporters in Tokyo, where he is on a visit.
"We need to be very careful about coming to hard and fast conclusions too soon."
Up to a dozen planes and 13 ships will be scouring three separate areas about 2,000 km (1,240 miles) northwest of Perth, Australia's Joint Agency Coordination Centre said on Sunday.
A black box detector deployed by Chinese 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 and 101 degrees east, Xinhua said.
"The 37.5kHz is the specific frequency that these locator pingers operate on," said Anish Patel, president of Sarasota, Florida-based Dukane Seacom, which made the black box locator.
"It's a very unique frequency, typically not found in background ocean noise," such as whales or other marine mammals, he told Reuters.
Xinhua also reported that a Chinese air force plane had spotted a number of white floating objects in the search area.
"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 from its set route.
Dozens of flights by a multinational taskforce have failed to turn up any trace of the plane in the past four weeks.
It was briefly picked up on military radar west 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.
Malaysian 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, Siva Govindasamy and Niluksi Koswanage in KUALA LUMPUR and Jane Wardell in SYDNEY; Editing by Mark Bendeich)