File photo taken on March, 2014 shows Chinese rescue vessel Nanhaijiu 115 on South China Sea.[Photo/Xinhua]

Search teams are still attempting to gain a signal from the airplane's Emergency Locator Transmitter.

The device is supposed to become active in the event of a crash.

Professor Huang Jun with Beijing-based Beihang University says there are reasons why a plane's emergency locator might not activate.

"First, the signal could be out of the reach of the search vessels. There are limits to how far the signal can transmit. The second possibility is that the signal is blocked by other objects. The other possibility is that the power to the transmitter is off."

On Monday, broad aerial surveys spotted two oily patches and objects in separate locations, but it's unknown whether any of it is related to the missing Airbus.

Authorities say the search area will be widened on Tuesday.

Most of the passengers on board were Indonesian.

Hong Kong authorities have confirmed that a resident of the city was onboard with his two-year-old daughter.