BUENOS AIRES — As Argentines paused Thursday to remember loved ones killed during a brutal dictatorship, President Obama said America was slow to speak out for human rights during that painful period and promised an honest accounting going forward.

His comments came 40 years to the day that a 1976 coup opened a period of military rule in Argentina known as the ‘‘Dirty War,’’ which lasted until 1983. At Remembrance Park, Obama said, “We’ve been slow to speak out for human rights, and that was the case here,’’ standing alongside Argentina’s new president, Mauricio Macri.

Obama said it takes courage for a society to address ‘‘uncomfortable truths’’ about its past, but it’s essential.

Human rights groups shunned an invitation to attend. Nora Cortinas of Mothers of the Plaza de Mayo said, ‘‘What would victims say if they saw [us] hugging and paying homage to a president from a country responsible for state terrorism?’’

Many families are still searching for the remains of loved ones. Some 13,000 people were killed or disappeared, according to government estimates. Rights groups put the number closer to 30,000.

Obama said his administration will endeavor to make amends by declassifying more documents that could shed light on what role the United States may have played. Thousands of documents were declassified in 2002, but they don’t paint a full picture of US actions at the time.

Associated Press