Reuters reported that investigators believe the southerly route — which stretches from Indonesia to the Indian Ocean — is the spot where MH370 may have either crashed or landed without detection.

“The working assumption is that it went south, and furthermore that it went to the southern end of that corridor,” the source was quoted as saying.

The latest information strengthens earlier points raised over the impossibility that flight MH370 had flown the northern arc now plied by searchers, which stretches from northern Thailand to the border of Kazakhstan and Turkmenistan in central Asia.

This, according to a former US intelligence agent earlier this week, is because the jumbo jet ferrying 239 people would have at the very least triggered military alarms of the numerous nations along the northern corridor.

Mike Morell said in an interview Monday that the passenger plane had most likely gone the southern route where it would have better chances of escaping radar detection.

“There are a lot of defence radars up there with China and India and the US and Afghanistan.

“So again, it is most likely the southern route,” the former deputy director of the US Central Investigation Agency (CIA) said on US broadcast programme, CBS This Morning.

But according to AFP earlier today, the first full day of search across the Indian Ocean by Australian and American aircraft has returned no sighting of MH370, which has been missing for nearly 12 days now.

Malaysia had earlier asked Australia to take charge of searches in the “southern vector” of the massive search and rescue operations, which now span a staggering 7.68 square kilometres across land and sea masses.

AFP reported that three long-range American, Australian and New Zealand surveillance aircraft were deployed today, searching an area of 305,000 square kilometres, while five merchant ships have also responded to aid the operations.

“Three ships have transited through the area, another is in the area today and a further ship is expected to arrive on Thursday afternoon.

“Neither the ships nor the aircraft have reported sighting anything in connection to the aircraft,” the Australian Maritime Safety Authority was quoted as saying.

Investigators have charted two divergent paths for MH370 after the aircraft was last traced to be airborne over the Indian Ocean after making an air turn-back 200 nautical miles off the coast of Kelantan, based on a final “electronic handshake” detected by satellite at 8.11am on March 8.

From the data, investigators used the plane’s available speed range to deduce that it could be in one of two corridors: a northern arc from northern Thailand to the border of Kazakhstan and Turkmenistan in central Asia, or a southern one from Indonesia to the southern Indian Ocean.

In the CBS interview, Morell also said the Boeing 777-200 would have faced great difficulty in maintaining an altitude below 5,000 feet needed to stay off radars, especially if it flying over land.

“I’m not an aviation expert but the 777 pilots I’ve talked with say that would be very, very difficult to do. So again, possible but not likely,” he said in response to reports that the plane could have hidden its trail by adopting a technique known as “terrain masking”.

Several central Asian nations that lie along the northern corridor have bolstered Morell’s view, saying none of their military radars have picked up signals from any unidentified aircraft encroaching on their airspace.

India had said it last Saturday, followed by Pakistan a day later and yesterday, Kazakhstan and al Qaeda militants, making it unlikely that a jumbo jet had flown off course along a northern route via Thailand.

Malaysian officials have also dismissed as speculation reports that the jet may have flown at low altitude to avoid detection.