Samantha Power, the U.S. ambassador to the United Nations, was traveling to a refugee camp in northern Cameroon when the child was struck Monday, she said hours later in Maroua, capital of the country's Far North Region.
The 8-year-old boy crossed the road without looking near the city of Mokolo, Cameroon's Ministry of Defense said, citing local journalists traveling with the convoy.
The ambassador was not in the car that struck the boy, the ministry said.
The child received immediate care from an ambulance traveling with the group but died shortly afterward, Power said.
The diplomat said she and other U.S. and Cameroonian officials visited the boy's family following the accident to "offer our profound condolences and to express our grief and heartbreak over what the family is going through."
Power is traveling with U.S. Department of Defense and U.S. Agency for International Development officials on a weeklong trip to Cameroon, Chad and Nigeria to underline American support for the campaign against Islamist terror group Boko Haram.
Fleeing Boko Haram
Power's convoy was headed to the Minawao camp, home to tens of thousands of the 2.5 million people displaced by Boko Haram in the region.
The Far North Region has been repeatedly struck by Boko Haram attacks
, with the Nigerian-based terror group increasingly using kidnapped girls to carry out suicide bombings
"In the refugee camp we visited with, virtually every family you encountered has some horrific memory of Boko Haram coming into their village -- whether that's Nigerians who have come across the border to Cameroon, or Cameroonians who've been attacked in their own homes here in this country," Power said in Maroua.
"Vivid memories of soldiers coming in, burning everything, stealing livestock, killing the men, abducting the girls, killing often the girls and the boys as well."
U.S. backs terror fight
Cameroon is part of an 8,700-member Multinational Task Force to fight the terrorists. The United States has contributed 300 troops as well as equipment to the effort.
"The United States stands with you, and we will look to support you economically as well as through providing intelligence and military and other support," Power told assembled Cameroonians on Monday.
In an annual report released in November, the Global Terrorism Index, Boko Haram was listed as the world's deadliest terror group
of the previous year, responsible for 6,644 deaths.
Last week, CNN exclusively obtained a "proof of life" video
of 15 of the 219 Nigerian girls missing since Boko Haram abducted them two years ago -- prompting renewed global anger at the Nigerian government's failure
to rescue the girls.