US Ambassador Nikki Haley, in Geneva for meetings of the UN Human Rights Council, also said a separate formal commission of inquiry should be set up to report on human rights violations in Kasai province.
A 16-year-old student and a 30-year-old man went on trial on Monday for the murder in Kasai of American Michael Sharp and Swedish-Chilean Zaida Catalán two months ago.
The United Nations has questioned the Kinshasa government's handling of the probe that led to the arrest of the two suspects, saying it was hastily done.
“After the tragic deaths of Michael Sharp and Zaida Catalán, they and their families deserve justice,” Haley said in a statement.
“We owe it to their legacy to end the human rights abuses being carried out by armed groups and the DRC government against the Congolese people.”
Haley called on countries to support the US push for a special investigation set up by UN Secretary-General Antonio Guterres and a commission of inquiry “into these horrific acts.”
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Last week, a coalition of 262 Congolese and nine international rights groups including Human Rights Watch and Amnesty International urged the Geneva-based rights council to set up the inquiry on the Kasai violence.
The region has seen a major spike in violence since September when government forces killed tribal chief and militia leader Kamwina Nsapu who had rebelled against President Joseph Kabila.
The unrest has claimed more than 400 lives and forced more than 1.2 million from their homes, according to UN figures. Unconfirmed local statistics put the number of dead as high as 3,000.
The UN has also reported finding 40 mass graves, which Sharp and Catalán were investigating when they were abducted and killed.
The rights council has set up similar commissions of inquiry to collect information on atrocities committed in Syria and North Korea.