PHNOM PENH, Cambodia

Making history twice within hours, President Barack Obama today became the first U.S. president to set foot in Cambodia, a country once known for its Khmer Rouge “killing fields.” He left behind flag-waving crowds on the streets of Myanmar, the once internationally shunned nation now showing democratic promise.

Unlike the visit to Myanmar, where Obama seemed to revel in that nation’s new hope, the White House made clear that Obama is only in Cambodia to attend an East Asia Summit and said the visit should not be seen as an endorsement of Prime Minister Hun Sen and his government.

Indeed, Obama’s arrival in Cambodia lacked the euphoria of his greeting in Myanmar, where tens of thousands of people lined the streets of Yangon to cheer the first American president to visit a country that until recently had long been isolated from the West. “You gave us hope,” Obama declared in Yangon.

In Phnom Penh, small clusters of Cambodians gathered in the streets to watch the motorcade pass by, without any of the outpouring that greeted Obama in Myanmar.

From the airport, Obama headed straight to the Peace Palace for a meeting with Hun Sen that later was described by U.S. officials as a tense encounter dominated by the president voicing concerns about Cambodia’s human rights record. He specifically raised the lack of free and fair elections, the detention of political prisoners and land seizures, officials said.

Deputy national security adviser Ben Rhodes said Obama told the prime minister that those issues are “an impediment” to a deeper relationship between the U.S. and Cambodia. Rhodes said Hun Sen defended his country’s record, saying unique circumstances motivate its policies and practices. Still, the prime minister expressed a desire to deepen ties with the U.S., Rhodes said.

Earlier in Myanmar, Obama addressed a national audience from the University of Yangon, offering a “hand of friendship” and a lasting U.S. commitment, yet a warning, too. He said the new civilian government must nurture democracy or watch it, and U.S. support, disappear.

The six-hour stop in Myanmar was the centerpiece of a four-day trip to Southeast Asia that began in Bangkok and ends Tuesday in Cambodia, where Obama will visit with Chinese, Japanese and Southeast Asia leaders in addition to attending the East Asia Summit with regional leaders.

Obama celebrated the history of what he was witnessing in Myanmar — a nation shedding years of military rule, and a relationship between two nations changing fast.

“This remarkable journey has just begun,” he said.

In a notable detour from U.S. policy, the president referred to the nation as Myanmar in his talks with President Thein Sein. That is the name preferred by the former military regime and the new government, rather than Burma, the old name favored by democracy advocates and the U.S. government.

Rhodes said afterward that Obama’s use of Myanmar was “a diplomatic courtesy” that doesn’t change the U.S. position that the country is still Burma.

On his first trip abroad since his re-election earlier this month, Obama’s motorcade sped him to the lakeside home in Yangon of longtime opposition leader Aung San Suu Kyi. He hugged her and lauded her as a personal inspiration. Suu Kyi spent most of the past 20 years in house detention at her home.

In remarks after their meeting, Suu Kyi echoed Obama’s tone with an admonition of her own, one that could have been directed at her own ruling party as much as to the United States:

“The most difficult time in any transition is when we think that success is in sight,” she said. “Then we have to be very careful that we’re not lured by the mirage of success.”

Rhodes said Obama was moved the visit with Suu Kyi at her home, and was pleased to see on prominent display a stuffed replica of the president’s dog Bo in the house. Obama gave Suu Kyi the stuffed animal when she visited Washington earlier this year.

Crowds swelled at every intersection in Yangon, yelling affectionately for both Obama and his secretary of state, Hillary Rodham Clinton.