Carlos Barria / Reuters President Barack Obama and Argentina's President Mauricio Marci throw flowers into the River Plate to honor victims of the Dirty War.

BUENOS AIRES, March 24 (Reuters) - President Barack Obama said the United States was too slow to condemn human rights atrocities during Argentina's 1976-1983 dictatorship as he honored victims of the "Dirty War" on Thursday, but he stopped short of apologizing for Washington's early support for the military junta.

Obama's state visit to Argentina coincided with the 40th anniversary of the coup that began a seven-year crackdown on Marxist rebels, labor unions and leftist opponents, during which security forces killed 30,000 people.

"There has been controversy about the policies of the United States early in those dark days," Obama said while visiting a memorial park in Buenos Aires dedicated to victims of the dictatorship.

"Democracies have to have the courage to acknowledge when we don't live up to the ideals that we stand for. And we've been slow to speak out for human rights and that was the case here," he said.

Obama's trip, winding up later on Thursday, is part of a wider effort to deepen ties and bolster U.S. influence with Latin America after years of frosty relations with left-leaning governments in the region.

With South America's leftist block now in disarray amid graft scandals and economic recession, Argentina's new center-right leader, Mauricio Macri, offers Obama a new ally in one of the Americas' biggest economies.

Obama traveled to Argentina from Cuba, where he became the first sitting U.S. president to visit in 88 years and opened a new chapter in engagement with the Communist-ruled island after decades of hostilities.

That policy shift has boosted Washington's standing in a region long wary of being treated as the U.S. "backyard," although U.S. foreign policy under Obama has still been dominated by the Middle East.

DEATH FLIGHTS

At the memorial on the banks of the La Plata River, Obama and Macri walked along a stark wall that is known as the Monument to the Victims of State Terrorism and is inscribed with 20,000 names.

In bright sunshine, they walked down to a pier that overlooks the river, dropping white roses into the water to commemorate the dead. Obama bowed his head and stood with Macri in silence.

Survivors of the crackdown say one of the military rulers' tactics was so-called "death flights," where political opponents were tossed into aircraft, stripped and then thrown alive into the river and the Atlantic Ocean to drown.

Washington's early support for the military rulers reflected Cold War thinking, which sometimes put the United States on the side of brutal right-wing governments in Latin America. In a gesture toward Argentines still angry over that legacy, Obama has promised to declassify U.S. military and intelligence records related to the dictatorship-era.

But the U.S. leader was criticized by some rights activists. One group of victims' relatives said the timing of his visit was a provocation.

"We will not allow the power that orchestrated dictatorships in Latin America and oppresses people across the world to cleanse itself and use the memory of our 30,000 murdered compatriots to strengthen its imperialist agenda," the Buenos Aires-based Center for Human Rights Advocates said in a statement.

Some Argentines welcomed Obama's gestures. "Obama is not going to say outright 'forgive us', but he's saying it through his actions," said Daniel Slutzky, a 75-year-old college professor.

Anadolu Agency via Getty Images

Obama said on Wednesday it was "gratifying to see Argentina champion our shared commitment to human rights." Yet Macri's opponents balk at the suggestion the socially conservative leader is a rights defender.

"It takes courage for a society to address uncomfortable truths about the darker parts of its past. Confronting crimes committed by your own leaders, by your own people - that can be divisive and frustrating, but it is essential to moving forward," Obama said.

Speaking after Obama, Macri said: "We have to reaffirm our commitment to the defense of democracy and human rights. Every day, somewhere in the world they are jeopardized."

Obama's visit to Argentina is a show of support for Macri's sharp turn away from the nationalist policies of his predecessor, Cristina Fernandez, who frequently railed against the United States and Wall Street. Obama praised Macri on Wednesday for his rapid economic reforms.

During his trip to Cuba, the U.S. president challenged President Raul Castro on human rights and political freedoms even as the two men sought to move on from more than half a century of animosity that began soon after Cuba's 1959 revolution.

Obama has been traveling with his family and later on Thursday they were to switch briefly into vacation mode, traveling to the lakeside town of Bariloche in Patagonia for the afternoon before returning to Washington. (Writing by Richard Lough and Hugh Bronstein; Editing by W Simon and Frances Kerry)