Buenos Aires (AFP) - President Barack Obama paid homage Thursday to victims of Argentina's former US-backed dictatorship, admitting the United States was "slow to speak out for human rights" in those dark days.

Obama became the first US president to formally acknowledge the victims of the 1976-1983 military regime, which declassified documents have revealed was supported by top US officials.

"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, and its own past," Obama said.

He spoke at Remembrance Park, a monument in Buenos Aires to the 30,000 people who were killed or went missing under the dictatorship. He paid tribute to victims' families.

"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 speak out for human rights. And that was the case here."

Tens of thousands of people joined a noisy demonstration later in Buenos Aires to mark the 40th anniversary of the US-backed coup that brought the dictators to power.

They marched to the din of drums, carrying pictures of the victims. Similar anniversary marches were called in towns across the country.

Some rights groups complained Obama had not gone far enough.

"The self-criticism was totally light," said Taty Almeida, founder of the victims' campaign group Madres Linea Fundadora.

She added that Argentine President Mauricio Macri and Obama "insisted we have to look to the future. They do not acknowledge the genocide and state terrorism that was supported by the United States."

- Coup anniversary -

Victims' groups had been angered by the choice of the date for Obama's visit, given the US support for the coup at the time.

But they welcomed his promise to declassify further documents to shed more light on the fates of the regime's victims.

After the memorial ceremony Obama with his wife Michelle, her mother and the couple's daughters flew to the Andean resort of Bariloche, where they went for a hike and boat ride in a national park.

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Locals lined the road smiling and waving as Obama's motorcade took the family from Bariloche airport, but at one place a crowd of protesters demonstrated noisily, some raising their middle fingers.

- Tango diplomacy -

In 2002, Washington declassified 4,000 diplomatic cables that showed US officials encouraged the Argentine junta's purge of leftists.

Obama promised to declassify other sensitive military and intelligence records linked to the "dirty war."

They may shed more light on US involvement in secret police operations against dissidents in other South American states including Chile, Uruguay, Bolivia and Brazil.

Obama has tried to present a softer side of US power in Latin America during the trip this week that took him first to Cuba and then Argentina.

In Havana, he attended a baseball match with Cuba's Communist President Raul Castro and even made an appearance playing dominoes in a television show with Cuban comedians.

In Buenos Aires, he joked about tasting Argentina's national beverage mate and trying to meet football superstar Lionel Messi.

He danced tango at a state dinner in the city on Wednesday.

- US creditors -

On the first bilateral visit by a US president to Argentina since Bill Clinton in 1997, Obama hoped to nurture a new regional ally.

He praised Macri for the economic reforms he has passed since taking office in December after 12 years of leftist rule by the late Nestor Kirchner and his wife Cristina.

Obama also welcomed Macri's "constructive approach" in reaching a deal with US creditors to settle debts dating to Argentina's financial crisis in 2002.

He said it had led to the "possibility of a resolution" that could let Argentina back into international financial markets.