President Obama expressed regret on Thursday for the failure of the United States to acknowledge the brutal repression and atrocities that took place during Argentina’s “dirty war” in the 1970s and ’80s.

“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.”