* Beach debris not from a Boeing aircraft - ATSB chief
* Malaysia steps up assets used for deep sea search
* Air, surface and undersea search continues (Updates details of search)
By Byron Kaye and Sonali Paul
PERTH/MELBOURNE, April 24 (Reuters) - Authorities ruled out any link between debris picked up on an Australian beach and a missing Malaysian jetliner on Thursday as a tropical cyclone again threatened to hamper a 26-nation air, surface and underwater search of the Indian Ocean.
The debris, found on Wednesday on a beach at the southern tip of Western Australia state, was seen as the first lead since April 4 when authorities detected what they believed was a signal from the black box of Malaysia Airlines flight MH370, which disappeared on March 8 with 239 people on board.
But it took Australian authorities less than a day to analyse detailed photographs of the beached debris, no description of which was given, and dismiss the possibility that it may be linked to the plane.
"We're not seeing anything in this that would lead us to believe that it is from a Boeing aircraft," Australian Transport Safety Bureau commissioner Martin Dolan the Australian Broadcasting Corp.
That puts the focus of the search, the most expensive in aviation history, back on U.S. Navy undersea drone Bluefin-21, which will soon finish scouring a 10 square kms (6.2 square mile) stretch of seabed where the acoustic pings were located.
Authorities have said if Bluefin-21 fails to find a trace of the plane in its initial target search area, some 2,000 kms (1,200 miles) northwest of the Western Australian city of Perth, it will be redeployed to new areas, still to be determined.
On Wednesday, Malaysian Transport Minister Hishammuddin Hussein told reporters that authorities would be "increasing the assets that are available for deep-sea search" and that his government was seeking help from state oil company Petronas which has expertise in deep-sea exploration.
Search authorities would need to "regroup and restrategise" if nothing was found in the current search zone, but the search would "always continue", Hussein said.
Australian search officials said weather conditions may impact the search effort after the air component was suspended for the previous two days because of heavy rain, strong winds, rough seas related to related to Tropical Cyclone Jack.
Up to 11 military aircraft and 11 ships were expected to help with the day's search although authorities would monitor the weather before the sorties commenced.
(Reporting by Byron Kaye and Sonali Paul)