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Eight Americans were among 157 people killed when a plane crashed shortly after takeoff in Ethiopia on Sunday morning.

The Ethiopian Airlines flight was headed from Addis Ababa en route to the Kenyan capital, Nairobi, with 149 passengers and eight crew members on board, the airline said. There were no survivors.

There were no immediate details on what caused the crash of the Boeing 737 Max 8 plane, which was new and had been delivered to the airline in November, but the Swedish flight-tracking website Flightradar24 said the plane "had unstable vertical speed" after takeoff.

The airline's chief executive, Tewolde GebreMariam, said at a news conference that the plane had no known technical issues and that the captain had an "excellent flying record," with more than 8,000 hours of experience.

He reported "difficulties" while in flight and wanted to return to the airport, GebreMariam said. He was given clearance to turn back, but it's unclear what happened next. The plane "was lost from the radar — it disappeared," he said.

Tewolde Gebremariam, the CEO of Ethiopian Airlines, looks at the wreckage of the plane. Ethiopian Airlines via AP

GebreMariam expressed his "profound sympathy and condolences to the families and loved ones of passengers and crew who lost their lives in this tragic accident" in a Facebook statement.

Ethiopia's House of People's Representatives declared Monday a national day of mourning.

Prime Minister Abiy Ahmed, who visited the site of the crash, offered his "deepest condolences" to the families of those on board.

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

President Uhuru Kenyatta also tweeted that he was saddened. "My prayers go to all the families and associates of those on board," Kenyatta said.

Authorities confirmed that the plane was carrying passengers from more than 30 countries. They included 18 Canadians; eight apiece from China, the United States and Italy; and seven each from France and Britain.

The Russian Embassy confirmed the deaths of three citizens and expressed its condolences, as did Canadian Prime Minister Justin Trudeau. Canada had the second-highest number of victims on the flight.

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"The Canadian government is in close contact with Ethiopian authorities to gather additional information as quickly as possible," said Chrystia Freeland, Canada's minister of foreign affairs.

No names were released. The airline said bodies would be delivered to the victims' families once their identities were determined.

The U.S. Embassy in Ethiopia said its officials were in contact with the country’s government and the airline "to offer all possible assistance." The U.S. State Department said its officials would contact family members of the U.S. passengers directly.

Four of those on board were listed as using United Nations passports, and their nationalities were not immediately clear. GebreMariam could not confirm whether the victims included U.N. staff and diplomats on their way to the U.N. Environment Assembly, which is taking place in Nairobi next week.

Maria Fernanda, president of the 73rd session of the U.N. General Assembly, tweeted her condolences, saying it’s "a popular route for many fighting for the good of Africa."

People look at the scene of the Ethiopian Airlines crash near the town of Bishoftu, southeast of Addis Ababa, on March 10, 2019. Tiksa Negeri / Reuters

Boeing said in a statement that a technical team would travel to the scene to provide assistance under the direction of the Ethiopia Accident Investigation Bureau and the U.S. National Transportation Safety Board, or NTSB.

"Boeing is deeply saddened to learn of the passing of the passengers and crew on Ethiopian Airlines Flight 302," it said. "We extend our heartfelt sympathies to the families and loved ones of the passengers and crew on board and stand ready to support the Ethiopian Airlines team."

The NTSB later said it would send a four-person team to assist in in the investigation because the aircraft was manufactured in the United States.

In October, another Boeing 737 Max 8 plunged into the Java Sea just minutes after taking off from Jakarta, Indonesia's capital, killing all 189 people on board.

The Addis Ababa-Nairobi route links East Africa's two largest economic powers and is popular with tourists making their way to safari and other destinations. Sunburned travelers and tour groups crowd the Addis Ababa airport's waiting areas, along with businessmen from China and elsewhere.

At the airport in Nairobi, worried families gathered.

"I came to the airport to receive my brother, but I have been told there is a problem," Agnes Muilu said. "I just pray that he is safe or he was not on it."

Family members of victims react to the news of a plane crash in Addis Ababa, Ethiopia, on March 10, 2019. Mulugeta Ayene / AP

The state-owned airline, widely considered the best-managed airline in Africa, calls itself Africa's largest carrier and has ambitions of becoming the gateway to the continent.

Sunday's crash comes as the country's reformist prime minister has vowed to open up the airline and other sectors to foreign investment in a major transformation of the state-centered economy.

Ethiopian Airlines has been expanding assertively, recently opening a route to Moscow, and in January it inaugurated a new passenger terminal in Addis Ababa to triple capacity.

Speaking at the inauguration, the prime minister challenged the airline to build a new "Airport City" terminal in Bishoftu, where Sunday's crash occurred.