Australian Prime Minister Tony Abbott told the world on Thursday that the search for Malaysia Airlines Flight 370 is "the most difficult search ever undertaken."

Abbott said the remark in a joint press conference with Malaysian Prime Minister Najib Razak, who said "the disappearance of MH370 has tested our collective resolve."

"I can assure you that the best brains in the world are working on this," Abbott said in the press conference. "As far as Australia is concerned, we are throwing everything we have at it." He called on the world to be patient as his country, and many others, continue to search for the vanished plane.

The search for Malaysia Airlines Flight 370 has spanned nearly four weeks since it disappeared on March 8 shortly after taking off from Kuala Lumpur International Airport. There were 239 people on board.

Ten aircraft from seven different countries contributed to the search over the southern Indian Ocean on Thursday, the Australian Maritime Safety Authority said in a tweet.

Ten aircraft from seven nations are flying sorties today in the #MH370 search area.

— AMSA News (@AMSA_News) April 2, 2014

Image: AMSA

The authorities are now focused on a search area of about 86,000 square miles of open ocean, about 1,000 miles north-west of Perth, Australia. It's a constantly changing target, the Australian government's Joint Agency Coordination Centre said:

The Australian Transport Safety Bureau continues to refine the area where the aircraft entered the water based on continuing ground-breaking and multi-disciplinary technical analysis of satellite communication and aircraft performance, passed from the international air crash investigative team comprising analysts from Malaysia, the United States, the UK, China and Australia.

To the families of those on board, many of whom refuse to believe that their loved ones are dead, Abbott said on Thursday, "I know this is an extraordinarily difficult time for you, but we will not let you down."

Should the time come to bring them to Australia, the prime minister said, referring to a point in the investigation that will only come once the plane's debris is found, he said the country would extend a warm welcome during this difficult period.

"This is one of those times where we are all citizens of one world," he concluded. "We are all global citizens."

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