Eight Americans were among the 157 people killed when an Ethiopian Airlines flight crashed shortly after takeoff Sunday from Ethiopia's capital, Addis Ababa.

The 4-month-old Boeing 737-8 MAX plane crashed six minutes into its flight to Nairobi, Kenya, plowing into the ground at 8:44 a.m. local time, 31 miles south of Addis Ababa.

The cause of the crash was not immediately known. The pilot sent out a distress call and was given clearance to return, said Tewolde GebreMariam, the airline’s CEO.

The new MAX 8 configuration – a single-aisle plane with room for up to 210 passengers – was certified for flight by U.S. and European regulators two years ago.

It has two major crashes on its record. In October, the same model plane operated by Lion Air plunged into the Java Sea minutes after takeoff from Jakarta, Indonesia. All 189 people on board were killed.

The plane operated by Ethiopian Airlines had routine maintenance Feb. 4 and had flown 1,200 hours, GebreMariam said. The pilot had nine years of seniority with the airline. Overall, the airline's safety record had been on par with other major world airlines.

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At least 35 nationalities, including 32 Kenyans and 18 Canadians, were among the dead, the airline said. The plane was carrying 149 passengers and eight crew members.

At least one of the passengers was carrying a United Nations passport, and the U.N. World Food Programme confirmed that multiple staffers were among those killed.

"The WFP family mourns today," Executive Director David Beasley said. "We will do all that is humanly possible to help the families at this painful time."

With a motto of "The New Spirit of Africa,” the government-owned airline is Africa's largest and has been aggressively expanding on the continent. It opened a new terminal at its hub in Addis Ababa in January, tripling its capacity.

The airline serves 61 cities in Africa and 45 more worldwide, including Chicago, Los Angeles, Washington and Newark, New Jersey. It is a member of Star Alliance, which gives it terminal and code-sharing agreements with partners such as United Airlines, Air Canada and Lufthansa.

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The airline's website crashed briefly Sunday as the airline tried to notify families of the dead.

Ethiopian Airlines published a photo of CEO GebreMariam standing in what appeared to be a crater caused by the crash, holding a piece of the wreckage. The photo shows the crash site on flat land under blue skies, and weather reports indicated clear visibility.

“The group CEO who is at the accident scene now regrets to confirm that there are no survivors,” the post on social media said. “He expresses his profound sympathy and condolences to the families and loved ones of passengers and crew who lost their lives in this tragic accident.”

The Ethiopian prime minister’s office offered its “deepest condolences” to families. “My prayers go to all the families and associates of those on board,” Kenyan President Uhuru Kenyatta said.

As worried families gathered at the airport in Nairobi, the scope of the tragedy slowly became clear.

“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.”

”Why are they taking us round and round? It is all over the news that the plane crashed,” said Edwin Ong’undi, who had been waiting for his sister. “All we are asking for is information to know about their fate.”

The Boeing 737 is the most popular airline passenger plane in the world; 9,600 have been built since 1967.

In a statement, Boeing said it was “deeply saddened” to hear of the crash and a technical team was ready to help at the request of the U.S. National Transportation Safety Board. The NTSB is sending a team of four investigators to Ethiopia, spokesman Eric Weiss said.

The Ethiopian Civil Aviation Authority is primarily responsible for the investigation, but the NTSB often provides technical assistance when an aircraft designed or manufactured in the USA is involved in a major crash anywhere in the world.

The last deadly crash of an Ethiopian Airlines passenger plane was in 2010 when an earlier-generation 737 crashed crashed minutes after takeoff from Beirut, killing all 90 people on board. The Lebanese Aviation Authority blamed crew error, but the airline disputed the findings and said there was an explosion before the crash.

Contributing: The Associated Press