Addis Ababa, Ethiopia (CNN) All 157 people aboard an Ethiopian Airlines flight that crashed soon after taking off Sunday morning from Addis Ababa have been killed, the airline said Sunday.

The plane, en route to Nairobi, Kenya, lost contact at 8:44 a.m. local time, six minutes after taking off from Bole International Airport in the Ethiopian capital.

Flight ET302 went down near Bishoftu, southeast of Addis Ababa. The victims were of 35 different nationalities, an airline spokesman told CNN.

Thirty-two Kenyans, 18 Canadians, nine Ethiopians, eight Americans, eight Italians and eight Chinese nationals were among the passengers. Seven French and seven UK nationals were also on board, the spokesman said.

Nineteen United Nations staff members were among those killed, according to a UN statement. The victims worked for the World Food Programme, the Office of the High Commissioner on Refugees, the International Telecommunications Union, the Food and Agriculture Organization, International Organization for Migration in South Sudan, World Bank and UN Assistance Mission in Somalia, and the UN Office in Nairobi.

Though it's unclear why UN employees were on the plane, the UN Environment Assembly is scheduled to begin Monday in Nairobi.

said. "My heartfelt condolences to the families and loved ones of all the victims — including our own UN staff — who perished in this tragedy," Secretary-General Antonio Guterres

A family member of a victim of the crash awaits news at Addis Ababa airport.

The US National Transportation Safety Board is sending a team to support the Ethiopian Accident Investigations Bureau's investigation, with assistance from technical advisers from the Federal Aviation Administration, Boeing and General Electric.

Ethiopian Airlines CEO Tewolde GebreMariam told reporters the pilot reported technical difficulties after takeoff and asked for clearance to return to Addis Ababa.

He was given clearance to turn back, GebreMariam said, citing the air traffic controllers' record.

The pilot was a senior Ethiopian Airlines pilot who had flown more than 8,000 hours. He had an "excellent flying record," GebreMariam said.

The CEO visited the crash site Sunday. He said the plane "is now right inside the ground" and it was not possible to identify whether it was an emergency landing or a crash. He said there was still smoke at the site when he visited.

"As it is a fresh incident, we have not been able to determine the cause. As I said, it is a brand new airplane with no technical remarks, flown by a senior pilot and there is no cause that we can attribute at this time."

"The routine maintenance check didn't reveal any problems," GebreMariam added.

Asked about the possibility of terrorism or sabotage, he said, "At this stage we cannot rule out anything."

The airline said a passenger information center and hotline "will be available shortly for family or friends of those who may have been on flight."

The Ethiopian government expressed its "deepest condolences to the families," the office of Prime Minister Abiy Ahmed tweeted.

The Office of the PM, on behalf of the Government and people of Ethiopia, would like to express it's deepest condolences to the families of those that have lost their loved ones on Ethiopian Airlines Boeing 737 on regular scheduled flight to Nairobi, Kenya this morning. — Office of the Prime Minister - Ethiopia (@PMEthiopia) March 10, 2019

Ethiopian Airlines has a reputation of being one of the best airlines in Africa. It has a good safety record and the newest fleet of planes on the continent, according to its website.

A Chinese group looks at the arrival flight schedule at Nairobi airport in Kenya on Sunday.

The Boeing 737 MAX 8 is the same type of plane as the Indonesian Lion Air jet that crashed soon after takeoff from Jakarta in 2018 -- killing 189 people, GebreMariam said, adding that only an investigation would yield conclusions.

Geoffrey Thomas, the editor in chief of Airline Ratings, told CNN that Sunday's crash had "significant differences" to the Lion Air crash. In the Lion Air flight, there were "wild fluctuations in air speed and ... we continued to get data from the plane all the way down to impact."

Family members of the victims react to news of the tragedy at Addis Ababa airport on Sunday.

Sunday's crash, however, had "no fluctuations and all of the sudden transmission" ceased, he said. "That transmission ceasing indicates catastrophic failure in air."

The aerospace giant said in a press statement that it was "deeply saddened to learn of the passing of the passengers and crew." It added that a "Boeing technical team is prepared to provide technical assistance at the request and under the direction of the US National Transportation Safety Board."

Boeing Statement on Ethiopian Airlines Flight 302: https://t.co/0jyiFuGHIE pic.twitter.com/NfE5S4LSlz — Boeing Airplanes (@BoeingAirplanes) March 10, 2019

The last major accident involving an Ethiopian Airlines passenger plane was in January 2010, when a flight from Beirut went down, killing 83 passengers and seven crew.