Family members of missing passengers are walking quietly into the conference hall at a Beijing hotel. An emergency rescue bed was wheeled in, followed by four emergency medical workers in orange uniforms.
Malaysia Airlines 370 "ended in the southern Indian Ocean," the prime minister said.
Malaysia Airlines has spoken to the family members of passengers and crew to inform them of this development, he added.
Information from Inmarsat, a British satellite company, and the UK Accident Investigation Branch -- which employed tactics "never before used in an investigation of this sort" -- indicates that the flight's last position was in the "remote" Indian Ocean, west of Perth, Australia, the prime minister says.
Despite news from Najib that the flight "ended" over the Indian Ocean, family members are still experiencing a "sense of uncertainty," as neither debris nor data and flight recorders have been found, according to CNN's Atika Shubert.
"This is a remote location, far from any possible landing sites. It is therefore with deep sadness and regret that I must inform you that, according to this new data, flight MH370 ended in the southern Indian Ocean," Najib said, adding that more details will be divulged during a Tuesday news conference.
Chinese families were called to a meeting room to listen to the briefing. The Chinese special envoy, Tan Sri Ong Ka Ting, was in the room, along with a former Malay transport minister. A Chinese woman walked out of the room crying, according to a stringer at the location.
A relative of a missing passenger exited Beijing’s Lido Hotel conference room where family members were meeting with Malaysia Airlines representatives and told CNN's Yuli Yang, "They have told us all lives are lost."
The UK Air Accidents Investigation Branch, which was mentioned by Najib, is declining to speak to media. It's referring media inquiries to Malaysian authorities. Attempts to reach Inmarsat, the satellite company mentioned, were not immediately successful.
Malaysia Airlines confirmed to CNN that it sent a text message to passengers' family members before formally briefing them in Beijing and Kuala Lumpur. The text said, "Malaysia Airlines deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived. As you will hear in the next hour from Malaysia's Prime Minster we must now accept all evidence suggests the plane went down in the southern Indian Ocean."
A senior administration official tells CNN the White House is aware of the prime minister's Malaysia Airlines announcement, and the administration will discuss it at an on-camera briefing at about 10:45 a.m. ET.
The airline is "working on an arrangement" to fly families to Australia if and when wreckage is found, a Malaysia Airlines spokesperson said.
Here is a statement Malaysia Airlines shared with the families of passengers and crew members:
"Malaysia Airlines deeply regrets that we have to assume that MH370 ended in the southern Indian Ocean. As you will hear in the next hour from Malaysia’s Prime Minister, new analysis of satellite data suggests the plane went down in the Southern Indian Ocean.
"On behalf of all of us at Malaysia Airlines and all Malaysians, our prayers go out to all the loved ones of the 226 passengers and of our 13 friends and colleagues at this enormously painful time.
"We know there are no words that we or anyone else can say which can ease your pain. We will continue to provide assistance and support to you, as we have done since MH370 first disappeared in the early hours of 8 March, while flying from Kuala Lumpur to Beijing.
"The ongoing multinational search operation will continue, as we seek answers to the questions which remain. Alongside the search for MH370, there is an intensive investigation, which we hope will also provide answers.
"We would like to assure you that Malaysia Airlines will continue to give you our full support throughout the difficult weeks and months ahead.
"Once again, we humbly offer our sincere thoughts, prayers and condolences to everyone affected by this tragedy."
Distraught relatives of MH370 passengers are being wheeled out in emergency beds from the Lido Hotel conference room in Beijing after learning the plane is believed to have been lost in the southern Indian Ocean.
One group of relatives smashed the lens of a media camera as they left the conference room.
New analysis of the Inmarsat "handshake" data concluding that Malaysia Airlines Flight 370 ended in the southern Indian Ocean was a groundbreaking math-based, peer-reviewed process revealing a direction of travel, said Chris McLaughlin, a spokesman for the satellite company. The analysis rules out a northern path for the plane, McLaughlin told CNN's Anderson Cooper.
Sarah Bajc, partner of American passenger Philip Wood, issued a statement saying, "The announcement is on data only, no confirmed wreckage so no real closure. i need closure to be certain but cannot keep on with public efforts against all odds. i STILL feel his presence, so perhaps it was his soul all along. ... i need to regroup. it looks like the first phase of our mission has ended. now philip's family and i will need some time for private grief."
Saying Inmarsat was "saddened for the families involved," the satellite company's spokesman, Chris McLaughlin, said he hoped the tragedy could yield useful change.
"The only thing you can hope is that from this, just as the Titanic resulted in the Safety of Lives at Sea law, that from this, there will be a mandate that all aircraft should be constantly tracked."
A woman came out of the briefing room at Beijing's Lido Hotel, screaming, "You announce this information today. Is it really confirmed? What's your proof? We've been waiting for 17 days. You simply tell us this. Where is the proof? It's wrong to announce the information like this. Where is the proof? You haven't confirmed the suspected objects to tell us no one survived. Where is your proof after 17 days? You simply tell us this. The Chinese government should come forward and clarify this. This happened on March 8. My mother passed away on March 9. How can I survive? The government is too dark."
The woman's reaction, translated by CNN's Yuli Yang, comes amid reports that relatives are reacting with sadness and anger to today's announcement by Malaysian Prime Minister Najib Razak that the plane went down in the Indian Ocean.
During a briefing at The Hague, Netherlands, White House deputy national security adviser Ben Rhodes said the announcement on Malaysia Airlines Flight 370 "tracks" with where the U.S. has deployed resources along the "southern corridor."
Rhodes said the U.S. is still seeking "independent confirmation" of the plane's whereabouts. President Barack Obama has been briefed daily on U.S. efforts to support the search for MH370 and expressed his condolences to Chinese President Xi Jinping today during their bilateral meeting in the Netherlands.
China’s Maritime Safety Administration is working on plans to ramp up search efforts and is looking at sending more ships to the southern Indian Ocean to look for plane wreckage, according to the official newspaper of the country's Ministry of Transport.
Chinese Foreign Ministry spokesman Hong Lei said in a brief statement on the ministry website, “China is aware of Malaysia’s announcement (of the plane crash). We are paying high attention to it. China has requested Malaysian authorities to further provide all information and evidence leading up to such conclusion. China’s search and rescue efforts are continuing. We also hope those of Malaysia and other countries could go on as well.”