Argentina: Obama visits 'Dirty War' memorial on coup anniversary
President Barack Obama has visited a memorial for the victims of Argentina's military dictatorship, on the final day of his visit to the country.
He promised to release more secret military and intelligence files from the era, revealing the US government's role in the 1976 coup.
The US was "too slow to stand up for human rights" in Argentina, he said.
Some 30,000 people are estimated to have been killed during the six years of military government.
Thousands of other people were illegally detained and tortured in what became known as the "Dirty War".
General Jorge Rafael Videla, who led the coup, died in jail in 2013, serving a sentence for human rights violations.
President Obama said US foreign policy had changed since to always take into account transparency and human rights.
He expressed hope that his gesture to release secret files of the 1970s would help mend relations between the two countries.
"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."
Later, tens of thousands of people marched through the streets the Buenos Aires to mark the 40th anniversary of the coup.
More than 4,000 secret files from the American government were released in 2002.
But on Wednesday, after a meeting with President Mauricio Macri, Mr Obama announced that for the first time the US had agreed to declassify military and intelligence files from the era.
After his visit to Argentina's Remembrance Park, on the banks of the River Plate, Mr Obama said he had had agreed to declassify more documents from that period.
"I believe we have the responsibility to confront the past with honesty and transparency," he said.
Mr Macri, said his country had to look to the future and build new relationships.
"This is a marvellous opportunity for all of the Argentine people to gather and say never again. Never again in Argentina to political violence, never again to institutional violence," said Mr Macri.
Mr Obama arrived in Argentina on Wednesday after a historic three-day visit to Cuba.
He praised the reforms implemented by Mr Macri's centre-right government since he came to power in December.
US-Argentina relations were less close during the previous left-wing governments of Mr Macri's predecessors, Cristina Fernandez de Kirchner and Nestor Kirchner.
President Macri said the visit marked the beginning of "new mature and intelligent relations" between the two countries.
Mr Obama has travelled to the southern Andean resort of Bariloche and returns to Washington on Thursday night.