It was a brief but symbolically powerful moment.

President Obama stepped alone into the frame of the Door of No Return on Senegal’s Goree Island on Thursday afternoon, peering out at the Atlantic Ocean from the same vantage point that thousands of African slaves once did on their way to North America.

The United States’ first African American president was then joined by his wife, first lady Michelle Obama. Their daughters, Malia and Sasha, also took a turn.

For Obama, the tour of the former slave house was one in a series of emotionally weighty visits through which the president intends to accentuate his bid to spread U.S. values and strengthen his administration’s ties with three budding African democracies during a week-long trip.

“This is a testament to when we’re not vigilant in defense of human rights what can happen,” he told reporters at the scene.

On Friday, the president will continue his push in South Africa, where he will spend three days, highlighted by a tour this weekend of the Robben Island prison where anti-apartheid icon Nelson Mandela spent nearly two decades as a political prisoner.

White House officials have said Obama will defer to the Mandela family’s wishes regarding a possible visit with the 94-year-old former president, who continues to battle a serious lung infection.

During a news conference in Dakar, Obama called Mandela a “hero” whose writings in defiance of South Africa’s apartheid system inspired Obama to a life of political activism when he was a 19-year-old Occidental College freshman three decades ago.

“I’ve had the privilege of meeting Madiba and speaking to him,” Obama said, using Mandela’s tribal name and referring to their 2005 meeting, when Obama was a senator. “I think he’s a hero for the world. And if and when he passes from this place, one thing I think we’ll all know is that his legacy is one that will linger on throughout the ages.”

In a statement on Thursday, South African President Jacob Zuma said Mandela’s health had improved from critical to stable, bringing some relief to South Africans whose concerns grew on Wednesday as Zuma abruptly canceled an official visit to Mozambique so that he could visit Mandela and confer with doctors.

But even as Zuma implied that Mandela’s health was improving, one of Mandela’s daughters suggested otherwise. “I won’t lie. It doesn’t look good,” Makaziwe Mandela told the South African Broadcasting Corporation. “Anything is imminent.”

Mandela, she said, was still giving the family a sense of hope because he was reacting to touch and opening his eyes. “We will live with that hope until the final end comes,” she said.

She and Zuma also took aim at the foreign media, accusing them of spreading rumors and publishing unconfirmed reports about Mandela’s condition. Zuma appealed for “respect for the privacy and dignity of the former president.”

Hundreds of journalists have arrived in South Africa, waiting outside Mandela’s hospital as well as his home in Houghton, an upscale Johannesburg neighborhood, and in Qunu, his ancestral village.

Makaziwe Mandela described the media as “vultures waiting when a lion has devoured a buffalo, waiting there for the last carcasses. That’s the image that we have, as a family.”

In Dakar, Obama used his personal reflections on Mandela to help push democratic values at a time when sub-Saharan Africa has emerged as one of the fastest-developing regions in the world and other countries, including China, have begun ramping up investment on the continent.

The president acknowledged that the United States has been neglectful, but he pledged to support countries such as Senegal, which Obama held up as an example to neighboring countries for its pursuit of a free society whose government is held accountable.

In a meeting with Senegalese President Macky Sall, Obama praised him for pursuing a case against former Chad leader Hissene Habre, whose administration is accused of killing thousands, for crimes against humanity. And the president pledged to support Senegal’s trial of Habre with resources, including a $1 million grant to the special court, administration officials said.

But the challenges that remain were apparent during a joint news conference when Obama hailed the U.S. Supreme Court’s decision this week that a law denying federal benefits to married same-sex couples is unconstitutional. Sall, asked by a reporter whether he would pursue changes to Senegal’s law that criminalizes homosexuality, said he did not think his country was ready for such a change.

“These issues are societal,” he said. “We should not have one standard model that’s applicable to all nations.”

Raghavan reported from South Africa.