Images posted on Twitter show the debris from the Lion Air crash.
A passenger plane reportedly carrying 189 people has crashed into the sea off Jakarta, minutes after take-off.
A search and rescue effort has been launched for the Boeing 737-800 plane which departed Jakarta about 6.20am on Monday.
"It has been confirmed that it has crashed," Yusuf Latif, a spokesman for the agency, said by text message, when asked about the fate of the plane.
Indonesia's search and rescue agency Basarnas said 189 people were on board the plane.
Basarnas chief Muhammad Syaugi said that Indonesian authorities were not yet able to say how many people had died.
He added that body parts had been seen floating near Tanjung Karawang, where the plane is believed to have gone down, about 34 nautical miles north-west of Jakarta.
"We don't know yet whether there are any survivors," Syaugi told a news conference, adding that no distress signal had been received from the aircraft's emergency locator transmitter.
"We hope, we pray, but we cannot confirm."
Of those on board, there were 178 adult passengers, one child, two babies and eight crew members, including the pilot and co-pilot.
Basarnas has sent out boats and helicopters to search for the plane and has found wreckage and lifejackets. About 150 rescuers, including 30 divers, have been sent to the scene.
Syaugi said the search and rescue teams had come from Jakarta, Bandung and Lampung and were headed to Tanjung Karawang.
The depth of the water where the plane is believed to have crash landed is 30 to 35 metres. The black box has not been located.
The head of Indonesia's disaster agency, Sutopo Purwo Nugroho, has tweeted images which he said showed debris and personal belongings that came from the aircraft.
He also shared a video he said had been taken from a tugboat off Karawang, just east of Jakarta, which appeared to show debris floating in the water.
"The plane crashed into water about 30 to 40m deep," Search and Rescue Agency spokesman Yusuf Latif told AFP news agency. "We're still searching for the remains of the plane."
An official of Indonesia's safety transport committee said he could not confirm the cause of the crash, which would have to wait until the recovery of the plane's black boxes, as the cockpit voice recorder and data flight recorder are known.
"We will collect all data from the control tower," said Soerjanto Tjahjono. "The plane is so modern, it transmits data from the plane and that we will review too. But the most important is the blackbox."
Lion Air Group's Chief Executive Edward Sirait earlier told Reuters: "We cannot give any comment at this moment. We are trying to collect all the information and data."
The New Zealand Embassy in Jakarta has had no requests for consular assistance into the plane crash.
A Ministry of Foreign Affairs spokesperson said Kiwis requiring consular assistance should get in touch with the Embassy.
"The New Zealand Embassy in Jakarta is monitoring the situation closely and is in contact with local authorities to determine whether any New Zealanders are affected," the spokesperson said.
"New Zealanders requiring consular assistance can call the New Zealand Embassy in Jakarta on (+ 62 21) 2995 5800."
Currently, 321 New Zealanders are registered on SafeTravel as being in Indonesia.
Earlier, a Lion Air spokesman Danang Mandala Prihantoro said "we can confirm that one of our flights has lost contact, its position cannot be ascertained yet."
Edward Sirait, chief executive of Lion Air Group said, "We cannot give any comment at this moment. We are trying to collect all the information and data."
Flight JT610 lost contact 13 minutes after takeoff, according to the official. The jet was a Boeing 737 MAX 8, which can carry up to 200 passengers, according to air tracking service Flightradar 24.
Reports received at Tanjung Priok port in North Jakarta indicate the remains of the plane had been spotted.
A vessel traffic officer, Suyadi, told the Jakarta Post that at 6.45am he received a report from a tugboat that thad identified a downed plane.
"At 7.15am the tugboat reported it had approached the site and the crew saw the debris of a plane," Suyadi told the Post. There was no reports about the passengers on board.
A tanker and cargo ship were approaching the site and a search and rescue vessel was also en route.
A telegram from the National Search and Rescue Agency to the air force has requested assistance with the search of a location at sea off Java.
Data processed by Flightradar shows an increase in speed and a decrease in altitude according to the last transmission from the aircraft.
Lion Air was set up 1999 and is a privatey owned based in Jakarta, Indonesia. The carrier operates an extensive network of domestic and international passenger and cargo services using mainly Boeing 737s.
The 737MAX was launched as a more fuel efficient version of one of the world's most popular single aisle planes.
Lion Air ordered 201MAX planes soon after the development programme got underway in 2011.
In 2007, Lion Air was one of several Indonesian airlines banned by the European Union for lax safety standards and that ban remained in place for nearly a decade.
In 2013, a 737-800 operated by the budget carrier crashed into the water near an airport in Bali. None of the 108 passengers or crew on board this week's crashed Bali were killed, though dozens were injured.
During the past week the downed aircraft had flown mainly within Indonesia but had several flights to China of up to seven hours.
The plane was powered by LEAP engines, made by CFM which has delivered more than 1000 of them to airlines in the past decade. The engines have racked up more than 1.5 million flight hours.
According to Flightradar the aircraft is a near new Boeing 737 MAX, a new generation plane.
It was delivered to Lion Air in August of this year.
More to come
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