“There’s been controversy about the policies of the United States early in those dark days,” Mr. Obama said at the Parque de la Memoria, a monument to the war’s victims, where he attended a ceremony for the 40th anniversary of the 1976 military coup that began the Argentine dictatorship.

The United States “has to examine its own policies as well, and its own past,” Mr. Obama added. “We’ve been slow to speak out for human rights, and that was the case here.”

The president’s remarks came after he toured the memorial with President Mauricio Macri of Argentina, walking beside a hulking gray stone wall engraved with the names and ages of 20,000 victims — plus 10,000 blank spaces for those who have yet to be identified.

Mr. Obama announced this week that he would begin a declassification effort to unseal secret military and intelligence files that could shed light on the fates of some of those victims, as well as what the United States knew about the human rights violations that took place during what Mr. Macri called “the darkest period in our history.”

The leaders walked together to a bridgehead overlooking the Río de la Plata, where they each cast three white roses into the water to honor the victims.

“A memorial like this speaks to the responsibilities that all of us have,” Mr. Obama said later. “We cannot forget the past, but when we find the courage to confront and we find the courage to change that past, that’s when we build a better future.”

Human rights groups had reacted angrily to the timing of Mr. Obama’s trip, arguing that it was inappropriate for him to visit at the very moment that Argentina was commemorating a tragic turn in its history that many believe was condoned, and in some cases enabled, by the United States.

Mr. Macri thanked Mr. Obama for participating in Argentina’s somber day of remembrance, and said nations must not be “passive onlookers” of human rights violations, as had been the case in the past.

“This is a marvelous opportunity for all of the Argentine people to say together, ‘Never again,’” Mr. Macri said. “Never again to institutional violence.”

Mr. Obama and his family then departed to tour Argentina’s scenic Patagonia region. He was being kept abreast of the investigation into theBrussels attacks by his national security staff back in Washington.

Lisa Monaco, his top counterterrorism adviser, briefed Mr. Obama by secure phone call Thursday morning, a White House official said. The president has directed his team to “continue providing any and all requested assistance to Belgian and other authorities investigating the attacks,” the official said.

The White House said Thursday that President Obama will meet with President Xi Jinping of China next week on the sidelines of a nuclear security meeting in Washington.

The two leaders, who last met in September, will discuss how they can work together on “issues of mutual interest,” and also to “address areas of disagreement constructively,” the White House said in a statement.

The meeting will take place on March 31.