KAMPALA, Uganda — The humanitarian group Doctors Without Borders said Wednesday that its staff saw Ethiopian government troops shooting and killing four men whom soldiers forced off a civilian bus on a road in the embattled region of Tigray.

Doctors Without Borders, or MSF, said in a statement that the killings on Tuesday followed the apparent ambush of an Ethiopian military convoy by an armed group. It said the MSF staff were able to witness the alleged extrajudicial killings because they were travelling ahead of two public mini-buses that were stopped by soldiers on the road from Mekele, the regional capital, to the city of Adigrat.

“The soldiers then forced the passengers to leave the mini-buses. The men were separated from the women, who were allowed to walk away. Shortly afterward, the men were shot,” the statement said, adding that the “horrific event further underscores the need for the protection of civilians during this ongoing conflict.”

There was no immediate reaction from the Ethiopian government.

Concern continues to grow over the humanitarian situation in the region that is home to 6 million of Ethiopia’s more than 110 million people. Authorities haven’t cited a death toll in the war.