Ethiopian Prime Minister Abiy Ahmed has declared Monday a day of mourning after dozens of aid workers, doctors, UN officials and World Food Programme staff were killed in a plane crash minutes after taking off from Addis Ababa airport.

The Ethiopian Airlines flight travelling from the country's capital to Nairobi crashed on Sunday morning into a vast plain of Ethiopian countryside, killing all 157 people on board.

The 149 passengers and eight crew members were on flight ET302, a Boeing 737-800 Max aircraft, when it took off from Bole International Airport heading to Jama International Airport in Kenya.

The plane was 62 kilometres south-east of Addis Ababa when it crashed near the town of Bishoftu, after repeatedly gaining and losing altitude.

Passengers from 35 countries were on board, including Britons, Kenyans, Americans, French and Chinese.

Many of the passengers were heading to a UN conference on the environment, where about 4,700 people were gathering in Nairobi on Monday.

At least one of the passengers was travelling on a UN passport.

The acting head of UN Environment, Joyce Msuya, said she was deeply saddened to hear the news of the crash.

The executive secretary of the UN Economic Commission for Africa, Dr Vera Songwe, said: "The United Nations family is mourning the loss of our colleagues and friends, many of whom lost their lives carrying out their professional duties today."

The organisers of the conference told The National they were in close communication with Ethiopia and Kenyan authorities.

Mr Abiy, who announced the news, offered his "deepest condolences to the families of those that have lost their loved ones".

He visited the crash site on Sunday and called for a "full and timely" investigation into the cause of the accident, a call repeated by Ethiopian Airlines and Boeing.

The flight took off at 8.38am local time from Addis Ababa before it lost contact only six minutes later.

The plane had "no known technical problems", Ethiopian Airlines group chief executive Tewolde Gebremariam said.

Mr Gebremariam said the pilot had an "excellent flying record" but had asked for permission to turn back shortly after take-off.

At Jomo Kenyatta International Airport, distraught family members and friends were being counselled by an emergency response unit.

One family avoided tragedy after a delayed flight from Dubai to Addis Ababa made Ahmed Khalid, who works in the emirate, miss his connection to Nairobi.

His father, Khalid Ali Abdulrahman, thought his son was on the ill-fated flight until he received a call from Mr Khalid saying he had arrived safely on a later service.

Many others were not so lucky. Austrian doctors, Italian aid workers, UN staff, World Food Programme workers and a Kenyan hotelier were among the dead.

A family member of a victim involved in a plane crash talks on a mobile phone at Addis Ababa international airport. AP

Mr Gebremariam was among the first to inspect the crash site and expressed "profound sympathy and condolences to the families and loved ones" of those who died, the airline said.

There was a deep scar in the earth where the plane crashed. Fragments of the aircraft littered the brown grass where hundreds of people gathered.

No large segment of the plane remained.

People watch as a tractor excavates the scene of the Ethiopian Airlines Flight ET 302 plane crash. Reuters

Emergency workers and crash scene investigators shifted through the mounds of metal, earth and victims' possessions.

Leaders from across the world were quick to release tributes to the families of the victims.

Sheikh Mohamed bin Zayed, Crown Prince of Abu Dhabi and Deputy Supreme Commander of the UAE Armed Forces, offered his sincere condolences Mr Abiy and the families of the victims.

We are saddened by the news of an Ethiopian Airlines passenger aircraft that is reported to have crashed 6 minutes after takeoff en route to Kenya. My prayers go to all the families and associates of those on board. — Uhuru Kenyatta (@UKenyatta) March 10, 2019

The current President of the UN General Assembly, Maria Fernanda Espinosa Garces, offered her "heartfelt thoughts" and said it was "a popular route for many fighting for the good of Africa".

Kenyan President Uhuru Kenyatta said he was praying for the families and associates of those on board the plane, while Rwandan President Paul Kagame said: "We stand with Prime Minister Abiy and the people of Ethiopia."

In a statement posted on their website, Boeing extended heartfelt sympathies to the victims and said it would support Ethiopian Airlines.

A full, joint investigation into the cause of the crash was announced by Mr Abiy, the Ethiopian Civil Aviation Authority, Ethiopian Airlines and Boeing.

The aircraft maker said it was sending a team to provide technical assistance.

The US National Transport Safety Board would send four people to assist at the site, its spokesman said on Sunday.

The US Federal Aviation Administration is also monitoring developments, it said.

Mr Gebremariam said the airplane had flown only 1,200 hours and was four months old.

It arrived in Addis Ababa from Johannesburg on Sunday morning and no problems were reported, he said.

There was no immediate information on what might have caused the crash, but the altitude of the plane was "unstable after take off", air-traffic monitoring site Flightradar24 said.

Ethiopian Airlines is a point of national pride, said Ahmed Soliman, a research fellow at Chatham House's Africa programme.

"If you come into Addis Ababa airport you have big signs for Ethiopian Airlines, big posters advertising the modernity of this airline," Mr Soliman said.

He said it had been a rapidly growing business with ambitions to become a five-star carrier "just behind those peak airlines such as Emirates and Etihad".

"It has been one of, if not the, most successful businesses in the country," Mr Soliman said.

The crash will be a blow to the state-owned airline but should not detract from its goal to be the "premier carrier on the African continent", Mr Soliman said.

Dozens of flights out of Addis Ababa's airport were delayed or cancelled on Sunday.

The last major crash involving Ethiopian Airlines was in January 2010 when a Boeing 737 caught fire five minutes after takeoff from Beirut International Airport, killing all 89 passengers and crew on board.

"This is the second fatal accident involving the Boeing 737 Max in less than five months, but we shouldn’t draw conclusions yet as search and rescue is still under way – and this remains a priority," Alex Macheras, an aviation analyst, told The National.

In October this year, a Lion Air-operated Boeing 737 Max crashed shortly after departing from Jakarta, killing all 189 people on board.

Investigators found that the pilots apparently struggled with an automated system designed to keep the planes from stalling.