White House aides say President Barack Obama will raise human rights concerns in his meeting today with Cambodia's long-time prime minister.
The 60-year-old Hun Sen has held power since Ronald Reagan was in the White House, and says he's not stepping down until he is 90.
He's been criticized for what some say is his violent, authoritarian rule.
President Barack Obama is opening his historic trip to Myanmar, becoming the first U.S. leader to visit the once-pariah nation.
Air Force One touched down in Yangon Monday morning. The president will meet with Myanmar Prime Minister Thein Sein and the country's revered democracy advocate, Aung San Suu Kyi (ahng sahn soo chee).
Obama will close his visit to Myanmar with a speech at Rangoon University, where he will praise the country's progress toward democracy but urge further reforms.
Obama has led efforts to reestablish ties with Myanmar following its move toward democracy following a half-century of military rule.
Some human rights groups, however, have criticized Obama's visit as a premature reward for a country that still holds political prisoners and is struggling to stem ethnic violence.