The United States said it was "deeply concerned" over a video purporting to show soldiers shooting dead 50 to 100 unarmed civilians in the Democratic Republic of Congo.
US State Department Acting Spokesman Mark Toner condemned the video footage released on Saturday, which appeared to show armed troops summarily executing civilians, including women and children.
"Such extrajudicial killing, if confirmed, would constitute gross violations of human rights and threatens to incite widespread violence and instability in an already fragile country," Toner said in a statement.
"We call upon the government of the Democratic Republic of Congo to launch an immediate and thorough investigation, in collaboration with international organizations responsible for monitoring human rights, to identify those who perpetrated such heinous abuses, and to hold accountable any individual proven to have been involved."
The apparent massacre took place in the village of Mwanza Lomba, amid ongoing clashes between government troops and Kamwina Nsapu militia members.
The DR Congo government has discounted the authenticity of the seven-minute video, calling it a "ridiculous fake... worthy of scenes from a Rambo movie."