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Search for Missing AirAsia Plane Expands on Third Day

"The hypothesis is the plane is at the bottom of the sea," Indonesia's National Search and Rescue chief Henry Bambang Soelistyo said.

A bevy of resources was added to the search efforts for AirAsia Indonesia Flight QZ8501 as the sun rose on the third day of hunting for the jet, which vanished Sunday morning with 162 people on board.

At least 30 ships, 15 aircraft and seven helicopters were now involved in the ever-expanding search in the Java Sea and nearby land, Indonesia's Search and Rescue Agency chief, Henry Bambang Soelistyo, told The Associated Press. The search area had increased from four “sectors” on Monday to seven on Tuesday, according to a statement from the Indonesian Search and Rescue Agency (BASARNAS).

Indonesia was leading the search, but Singapore, Malaysia and Australia were also providing resources, the statement said.

Singapore was flying C-130 military aircraft and had donated two sets of underwater locator beacon detectors — which can help find an aircraft's black boxes — and two teams of specialists, the country's civil aviation authority said.

On Monday, the United States received a request from Indonesia for help in the search, State Department spokesman Jeff Rathke said. The USS Sampson will travel to the search zone from the South China Sea, U.S. defense officials said Monday. The destroyer should arrive later in the day Tuesday, according to the U.S. Navy. Pentagon press secretary John Kirby said the U.S. might also provide air and sub-surface detection assets. "We stand ready to assist in any way possible," Kirby said. National Transportation Safety Board and Federal Aviation Administration officials have said they were standing by to provide assistance.

AirAsia Indonesia Flight QZ8501 lost contact with air traffic controllers Sunday after requesting a course change due to weather during a flight from Indonesia to Singapore. Officials have said there was no distress call from the cockpit.

"Until now, we have not yet found any signal or indication of the plane's whereabouts," Soelistyo told the AP, adding fishermen from Belitung island were also helping.

Soelistyo told reporters Monday that the plane likely crashed into the sea. "Based on the coordinates that we know, the evaluation would be that any estimated crash position is in the sea, and that the hypothesis is the plane is at the bottom of the sea," he said.

There were reports that an Australian aircraft on Monday had spotted "suspicious" objects about 700 miles from where the airliner lost contact, according to the AP. Jakarta's Air Force base commander Rear Marshal Dwi Putranto told the AP that it was too early to tell whether the objects were connected to the missing plane. NBC News could not independently confirm the reports.

Authorities have been careful to stress that the scenario for the missing AirAsia plane is drastically different from that of Malaysia Airlines MH370, which went missing in March and has yet to be found.

Australian Prime Minister Tony Abbott said "it would be a big mistake to equate what has happened here with MH370," telling Sydney's Radio 2GB that MH370 was "one of the great mysteries of our time."

The area of the Java Sea where the AirAsia plane went missing is typically 40 to 50 meters (130 to 164 feet) deep — compared with the 4,000 meter-plus depths complicating the MH370 search, U.K.-based oceanographer Dr. Simon Boxall told NBC News.

"It's relatively shallow water and it's close to land so they can use helicopters, rather than spending hours just getting to the search area," said Boxall, who is based at at the University of Southampton. "It's not going to be easy, but it is feasible within a relatively short amount of time."

The Airbus A320-200, which was travelling from Surabaya to Singapore, had 155 passengers as well as two pilots and five crew members on board. Most of the passengers are Indonesian. Three people on board are from South Korea, and one person each is from Singapore, Malaysia, Britain and France. Seventeen passengers are children, and one is an infant, the airline said.

Relatives of many of the passengers were flocking to the Singapore Changi Airport, where the missing plane was supposed to arrive, for updates.

AirAsia CEO Tony Fernandes has called the disappearance "a nightmare." He told reporters that his thoughts are with those who have loved ones on board.

"Until today, we have never lost a life," Fernandes told reporters in Jakarta airport, according to the AP. "But I think that any airline CEO who says he can guarantee that his airline is 100 percent safe, is not accurate."

The plane was supposed to arrive at Changi at 8:30 a.m. Singapore time (7:30 p.m. ET Saturday). The pilot had asked for permission to climb from 32,000 feet to 38,000 feet to avoid clouds, said Joko Muryo Atmodjo, air transportation director at Indonesia's transport ministry, according to Reuters. Permission was not granted.

Cassandra Vinograd, The Associated Press and Reuters contributed to this report.