By Byron Kaye
PERTH, Australia, April 23 (Reuters) - Australia 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.
Australian Prime Minister Tony Abbott admitted the search strategy may change if seabed scans taken by a U.S. Navy drone failed to turn up a trace of Malaysian Airlines flight MH370, 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, possibly on Wednesday.
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.
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 Malaysian Defence Minister Hishammuddin Hussein, 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 Joint Agency Coordination Centre said in a statement, adding 12 ships would continue to help with the operation.
Meanwhile, the Bluefin-21 was nearing the end of its first assignment scouring a 10 square kms (6.2 square mile) stretch of seabed where authorities traced what they believed was a black box signal two weeks ago.
Search officials have said that once the Bluefin-21's current mission, some 2,000 kms (1,200 miles) north west of the Australian city of Perth, is finished, they will redeploy the submarine to other areas yet to be determined.
(Additional reporting Matt Siegel in Sydney; Editing by Michael Perry)