US President Barack Obama visited a memorial commemorating victims of Argentina's military dictatorship, during his visit to the country.
The US is widely believed to have been involved in the coup which led to the dictatorship, under which an estimated 30,000 people were killed by the state.
Promising that the US would release more secret files to reveal its role in the 1976 coup, Mr Obama said the US was "too slow to stand up for human rights" in Argentina.
"There`s been controversy about the policies of the United States early in those dark days, and the United States when it reflects on what happened here has to examine its own policies as well, its own past," he said.
"Democracies have to have the courage to acknowledge when we don't live up to the ideals that we stand for.
"When we've been slow to stand up for human rights, and that was the case here."