The jet was flying at the regular cruising altitude for most jetliners - 32,000 feet - when the signal was lost, according to flight tracking website Flightradar24. No distress reports came from the plane, officials said.

Photos: Search for AirAsia Flight 8501

The aircraft had undergone its last scheduled maintenance on November 16, 2014.

The flight manifest for the jet showed there were 155 passengers on board plus seven crew members, although the Indonesian transport ministry only reported six crew members.

Of the passengers, 149 were Indonesians, three were South Korean, and there was one traveler each from Malaysia, Singapore and the United Kingdom, according to AirAsia.

AirAsia said the crew members consisted of six Indonesians and one person from France.

\"AirAsia
ABC News
AirAsia Airbus A320-200 Map flight path with times
>

The Civil Aviation Authority of Singapore said it was informed about the missing plane by Jakarta ground control about half an hour after contact was lost.

The Singapore air force and the navy were activated with two C-130 ready to assist Indonesia authorities in the search, according to the aviation authority.

\"We remain ready to provide any assistance to support the search and rescue effort,\" the CAAS said.

Saddened to hear of missing flight #QZ8501. My thoughts are with the passengers and their families. - LHL

— Lee Hsien Loong (@leehsienloong) December 28, 2014

A waiting area was set up at Changi Airport in Singapore, where the plane had been scheduled to land, with facilities and support for relatives and friends of the passengers on the plane. At Juanda International Airport in Surabaya, dozens of relatives sat in a room, many of them talking on mobile phones and crying. Some looked dazed, the Associated Press reported.

The White House said President Obama was briefed on the flight. A State Department spokesperson said the U.S. is \"ready to assist in any way that's helpful.\"

This is the third air incident this year for Malaysian-based airlines.

\"Relatives
Andy Wong/AP Photo
Relatives of passengers onboard the Malaysia Airlines Flight 370 that went missing on March 8, 2014, watch TV news about missing AirAsia flight QZ8501, during their year-end gathering at a house in Beijing, China Sunday, Dec. 28, 2014.
>

In July, another Malaysia Airlines jet was shot down over Ukraine, killing all 298 on board.

Very sad to hear that AirAsia Indonesia QZ8501 is missing. My thoughts are with the families. Malaysia stands ready to help.

— Mohd Najib Tun Razak (@NajibRazak) December 28, 2014

In March, Malaysia Airlines flight 370, a wide-bodied Boeing 777 with 239 people on board, lost contact with air traffic control soon after taking off from Kuala Lumpur for Beijing. It hasn't been found.

Ganyard lamented the disappearance of another aircraft this year.

\"It's really disheartening,\" said Ganyard. \"It just shouldn't happen in this day and age.\"

Get real-time updates as this story unfolds. To start, just \"star\" this story in ABC News' phone app. Download ABC News for iPhone here or ABC News for Android here. To be notified about our live weekend digital reports, tap here.

With reporting by ABC News' Ben Gittelson and Gillian Mohney.

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