Malaysian Prime Minister Najib Razak joined his Australian counterpart Tony Abbott Thursday in an inspection of the air force base in Perth that is serving as the centre of a huge international operation to find missing flight MH370.

Welcoming Mr. Najib to Pearce Air Force base north of Perth, the search coordinator, former Air Force chief Angus Houston, said the hunt for the missing Malaysia Airlines airliner was “one of the most complex operations the world has ever seen.” Mr. Najib received a personal briefing on the operation under way in the southern Indian Ocean, where the plane is believed to have crashed.

Australian PM Tony Abbott called this search is “the most difficult in human history”, while reiterating his promise to the families of those on board that his country will continue the current search process as best as they can.

Mr. Abbott told reporters this was probably the most difficult search ever undertaken, but “as far as Australia is concerned, we are throwing everything we have” to search the aircraft, Xinhua reported.

He also asked the families of those on board the lost jet to be “patient”, saying Australia and the multinational forces will not let them down and would provide the warmest possible welcome if they wanted to come to Australia.

Mr. Houston added that families of the 239 people on board the lost flight MH370 were welcome to visit the search headquarters in Perth.

The focus shifted Thursday to the north of Wednesday’s search zone, and aimed to cover 223,000 square kilometres, 1,680 kilometres west of Perth.

Eight planes and nine ships are involved, with a British nuclear submarine HMS Tireless joining the search along with UK search vessel HMS Echo.

Seeking to dampen expectations the search will be successful before the plane’s black box battery runs out in less than a week, Houston said it takes a long time to find ships and planes at the bottom of the ocean.

He noted it took 60 years to find the huge cruiser HMAS Sydney sunk in World War Two, even after witnesses saw the ship exploding over the horizon.

Malaysian police on Wednesday ruled out any of the passengers as suspects in the disappearance of flight MH370 on March 8, but said the investigation into the cabin crew, in particular the pilots, was ongoing.

Malaysian police chief Khalid Abu Bakar said the mystery of what happened on MH370 may never be solved.

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