(CNN) — A China Eastern Airlines jetliner carrying 132 people crashed in the mountains in southern China’s Guangxi region on Monday afternoon, according to the country’s Civil Aviation Administration (CAAC).

The Boeing 737 was en route from the southwestern city of Kunming to Guangzhou when it lost contact over the city of Wuzhou. On board were 123 passengers and nine crew members, CAAC said in a statement posted online.

China Eastern Airlines confirmed those details and said it had activated emergency procedures, including a line for emergency assistance for family members.

Rescue efforts are underway at the scene of the crash, but there were no immediate details on the possible cause or the number of casualties.

China Eastern offered its condolences to those who were killed in the incident, without confirming any death toll.

“The cause of the plane crash is still under investigation. The company expresses its sorrowful condolences to the passengers and crew members who died in this plane crash,” the airline said in a statement.

Boeing said in a statement: “Our thoughts are with the passengers and crew of China Eastern Airlines Flight MU 5735. We are working with our airline customer and are ready to support them.”

The company added that it is in contact with the US National Transportation Safety Board (NTSB) and that its technical experts are ready to assist in the CAAC investigation.

Later Monday, the NTSB said it “appointed a senior air safety investigator as a U.S. accredited representative to the investigation,” which will be led by the CAAC.

Boeing, engine manufacturer CFM, and the US Federal Aviation Administration will also be involved in the probe, the NTSB said. This arrangement is standard for aviation incidents involving a US-designed Boeing aircraft that occur in other countries.

Chinese President Xi Jinping instructed the country’s emergency services to “organize a search and rescue” operation and “identify the causes of the accident,” state media reported.

“After the accident, President Xi Jinping immediately made instructions to start the emergency mechanism, organize search and rescue, and properly deal with the aftermath,” state broadcaster CCTV said.

Sudden descent

The aircraft lost contact with emergency services before “suddenly descending” around 2:19 p.m., Chinese government officials and state media reported Monday.

“A China Eastern plane (flight number MU5735) lost contact at 2.15 pm … Rescue teams are on the way to ground zero, and rescue work is being laid out in order,” the Guangxi Emergency Management Department said in a statement.

The plane’s altitude dropped from 8,869 meters (29,098 feet) to 1,333.5 meters (4,375 feet) in the span of three minutes, state news agency China News Service reported, citing VariFlight, a Chinese technology company that provides civil aviation data services.

Hours after the accident, CCTV reported that the airline was grounding all its Boeing 737-800s and that the aircraft currently in the air would “not carry more flights after landing.”

CCTV also reported that rescue efforts could be hampered by bad weather and limited accessibility to the site.

Heavy rescue equipment was unable to reach the scene — which lacks electricity — as it is surrounded by mountains on three sides and accessible only through a narrow path, CCTV said, citing the Guangxi Wuzhou fire department.

Separately, Guangxi Meteorological Bureau warned that the rescue effort could be hindered by an incoming cold front that would see heavy rainfall and a temperature drop in Tengxian County, where the crash site is.

Eyewitness describes falling plane

In an interview with state media outlet Beijing Youth Daily, an eyewitness described seeing a plane “falling directly from the sky in front of him around 2 p.m.”

“The plane fell vertically from the sky. Although I was very far away, I could still see that it was a plane. The plane did not smoke during the fall. The fire started after it fell into the mountain, followed by a lot of smoke,” the witness, who was only identified by his surname, Liu, said.

“My heart was thumping. I immediately informed friends about the situation, that this area is dangerous and not to come nearby,” Liu continued.

In a separate interview with China News Service, a resident from Molang village in Tengxian county — close to the scene of the crash — reported seeing “wings and pieces of the plane, as well as pieces of clothing hanging from trees.”

The witness — whose name was not published — told state media he drove his motorcycle to the crash site after hearing “a huge explosion” around 2:40 pm to “see if he could participate in the rescue.” The onlooker added that the accident caused “about 10 acres of fire,” according to his visual estimates.

Video showing what appears to be a plane falling nose first from the sky circulated widely on Chinese social media Monday, before being picked up and published by state media.

Footage posted online and shared by state media outlet People’s Daily show plumes of smoke billowing from a mountainous, forested area. Another clip shows what appears to be wreckage from the plane on a muddy mountain path.

The colors on the Boeing and China Eastern Airlines websites were changed to black and white in China, as a sign of respect in response to the crash.

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