* Australia PM says the nature of the search for the missing plane could change
* Malaysian minister says authorities may "regroup and restrategise" if nothing found
* Authorities investigate find of debris off southern tip of Western Australia (Adds debris found on Australian coast, Malaysia minister comments, investigation approved)
By Byron Kaye
PERTH, Australia, April 23 (Reuters) - Australia and Malaysia vowed on Wednesday to keep searching for a missing Malaysian plane despite no sign of wreckage after almost seven weeks, and as bad weather again grounded aircraft and an undersea drone neared the end of its first full mission.
But Australian authorities said unidentified material washed up on the coast of Western Australa was being investigated for possible links to missing Malaysian Airlines Flight 370.
Australian Prime Minister Tony Abbott had earlier acknowledged that the search strategy could change if seabed scans taken by the U.S. Navy drone failed to turn up a trace of the plane, which vanished on March 8 with 239 people on board.
"We may well re-think the search but we will not rest until we have done everything we can to solve this mystery," he said.
"The only way we can get to the bottom of this is to keep searching the probable impact zone until we find something or until we have searched it as thoroughly as human ingenuity allows at this time."
The Bluefin-21 drone, a key component in the search after the detection of audio signals or "pings" believed to be from the plane's black box flight recorder, is due to end its first full mission in the southern Indian Ocean within days.
The finding of unspecified material on the southern tip of Western Austraia was the first report of suspected debris in weeks and the first since the detection of what were believed to be signals on April 4.
Australian police have secured the material, found 10 km (six miles) east of the town of Augusta, the Joint Agency Coordination Centre (JACC) overseeing the search said in a statement.
No description of the material was provided and Malaysian officials said they had not verified the report.
The Australian and Malaysian governments are under growing pressure to show what lengths they are prepared to go to in order to give closure to the grieving families of those on board flight MH370.
Malaysian acting Transport Minister Hishammuddin Hussein said the search authorities would need to "regroup and restrategise" if nothing was found in the current search zone, but said the search would "always continue".
"I can confirm in fact we are increasing the assets that are available for deep-sea search," he told a news conference in Kuala Lumpur, adding that the government was seeking help from state oil company Petronas, which has expertise in deep-sea exploration.
Nearly two months after the jetliner vanished, Hishammuddin announced that Malaysia's cabinet had approved the formation of an independent international investigation team to probe the causes of the baffling incident.
The experts appointed to the panel would be named next week, and the investigation could begin a week later, Malaysian officials said.
"It's imperative for the government to form an independent team of investigators which is not only competent, transparent but also highly credible," Hishammuddin said. "As I've said since the beginning, we have nothing to hide."
Malaysia's Department of Civil Aviation will take the lead in the investigation to be carried out under United Nations rules, sources have told Reuters.
But its relative lack of experience means it is likely to rely on foreign agencies, with some experts predicting a key role for Australia, which is coordinating search efforts.
Malaysian police are conducting their own criminal investigation into why the Boeing jet veered thousands of miles off course on a routine flight from Kuala Lumpur to Beijing.
In a sign of the families' growing desperation for answers, a group purporting to be relatives of the missing flight's passengers published a letter to Hishammuddin, urging the government to investigate old media reports that the plane landed in Kandahar, Afghanistan.
"It is high time that the government should start thinking out of the box by exploring and re-examining all leads, new and old," said the letter, published on Facebook on Wednesday.
Authorities suspended the air search for the second day in a row on Wednesday due to heavy rain, low cloud and big seas.
"Current weather conditions are resulting in heavy seas and poor visibility and are making air search activities ineffective and potentially hazardous," the JACC said, adding 12 ships would continue to help with the operation.
The Bluefin-21 was scouring a 10 sq km (4 sq mi) stretch of seabed some 2,000 km (1,200 miles) northwest of the Australian city of Perth. Search officials have said that once its current mission is finished, they will redeploy the submarine to other areas yet to be determined. (Additional reporting Matt Siegel in Sydney, Al-Zaquan Amer Hamzah and Yantoultra Ngui in Kuala Lumpur; Editing by Stuart Grudgings and Ron Popeski)