Media playback is unsupported on your device Media caption Air Chief Marshal Angus Houston described the search as "very complex, very demanding"

The search for missing Malaysia Airlines flight MH370 is the "most challenging" ever seen, the man responsible for co-ordinating the search from Australia says.

Air Chief Marshal Angus Houston also said that the search for the plane could take weeks.

The plane disappeared on 8 March as it was travelling from Kuala Lumpur to Beijing. It was carrying 239 people.

Search teams are scouring the southern Indian Ocean for signs of the plane.

A British submarine, HMS Tireless, has now joined the hunt. The vessel's ability to listen deep underwater could help find the plane's "black box" flight recorder, BBC defence correspondent Jonathan Beale says.

On Tuesday, the Malaysian authorities released the full transcript of communications between flight MH370 and Kuala Lumpur's air traffic control. They said there was no indication of anything abnormal in the transcript.

Flight MH370: Full transcript "Good Night Malaysian Three Seven Zero"

Full cockpit-tower communication [60K] Most computers will open PDF documents automatically, but you may need Adobe Reader Download the reader here

Speaking on Tuesday, ACM Houston, who is heading a new Joint Agency Co-ordination Centre (JACC) managing the search, said the task was "very complex" because the teams had no hard information to work from.

Search efforts would take time, he said. "It's not something that will necessarily be resolved in the next two weeks, for example."

Ten military aircraft and nine ships were scheduled to examine Tuesday's search zone, while an Australia defence vessel with a towed pinger was en route to the area, he added.

ACM Houston explained that they had no information on how high the plane had flown once it disappeared off radar, the BBC's Jonah Fisher in Perth reports.

A relatively small change in altitude could affect both the plane's speed and fuel consumption - and over the course of seven hours dramatically alter the location of any crash site, our correspondent adds.

Media playback is unsupported on your device Media caption Johan Fisher visited RAAF Base Pearce, which is at the heart of the search

Mistrust

It is now more than three weeks since flight MH370 disappeared. Malaysian authorities say that based on satellite data they have concluded that it crashed into the southern Indian Ocean.

MH370 - Facts at a glance 8 March: Malaysia Airlines Kuala Lumpur-Beijing flight carrying 239 people disappears

Malaysia Airlines Kuala Lumpur-Beijing flight carrying 239 people disappears Plane's transponder, which communicates with ground radar, was switched off as it left Malaysian airspace

Satellite 'pings' indicate plane was still flying seven hours after satellite contact was lost

24 March: Based on new calculations, Malaysian PM says "beyond reasonable doubt" that plane crashed in southern Indian Ocean with no survivors What we know The search for flight MH370

However, many relatives of the passengers on board have demanded proof that the plane has crashed, and expressed anger at what they perceive as a lack of information from the Malaysian authorities.

Dozens of relatives of some of the 153 missing Chinese passengers have travelled to Kuala Lumpur in their search for answers.

Late on Monday, Malaysian officials issued a new version of the last communication between air traffic control and the plane's cockpit.

In a statement, authorities said the last words received by ground controllers were "good night Malaysian three seven zero".

They had previously said that the last words from the plane were "all right, good night".

It is not clear why the official account has changed.

Correspondents say many family members of those on board have already been accusing officials of mishandling the search, and the latest change may add to their mistrust of the Malaysian authorities.