Air Asia QZ8501 flight search
Navy soldiers check a helicopter on the deck of KRI Sultan Hasanuddin-366 warship before joining in search operations for AirAsia flight QZ8501 at Batuampar port in Batam.Reuters

Indonesia is widening the search zone in the hunt for the missing AirAsia flight QZ8501 on the third day of its disappearance, as the vessels of US and China are also set to join the search.

The Airbus A320-200 passenger aircraft is presumed to have crashed in shallow Indonesian waters. Thirty ships, 15 aircraft and seven helicopters from Indonesia, Australia, Singapore and South Korea continue to scour the region to locate the crash site.

The search zone has been widened beyond the islands of Sumatra and Borneo in the Java Sea and authorities are also probing the island in East Bilutung, where reports of smoke have emerged.

Indonesian officials said the flight is likely to be at the bottom of the sea. "Until now, we have not yet found any signal or indication of the plane's whereabouts," said the chief of National Search and Rescue Agency's chief Henry Bambang Soelistyo.

He said the expanded search zone covers about 156,000 square kilometres. The Indonesian army has also been mobilised to carry out ground search for the aircraft.

The pilot's last contact with ground officials was to make a request to divert from its usual route due to bad weather. What exactly happened after that is still unknown.

Meanwhile, China has said it is sending its frigate and air force to assist the ongoing operations. "After we received the necessary clearance from the Indonesian government, the People's Republic of China has decided to dispatch its fleet to assist in the search of the missing aircraft," said Beijing's ministry of national defence, adding that one of its warships currently patrolling in the South China Sea is being redirected to the Indonesian waters.

In addition to that, the US has also said it is sending the USS Sampson guided missile destroyer, which is stationed in the western Pacific region, for the search. Pentagon press secretary Rear Admiral John Kirby said in a brief statement that the US assistance is taking place at the request of Indonesian authorities.

Though the details are still being examined, the US support "could include air, surface and sub-surface detection capabilities." He said: "We stand ready to assist in any way possible."