The State Department's second-ranking diplomat flew to Myanmar in May to urge the country's leaders not to adopt a tough "population-control" law apparently aimed at halting growth of persecuted ethnic minorities.
President Thein Sein listened politely to Deputy Secretary of State Antony Blinken. And hours after Blinken departed Yangon, he signed the bill into law.
Myanmar's leaders have repeatedly rebuffed U.S. appeals this year despite a public commitment to reform that led the White House to restore full diplomatic relations in 2012, and to drop most sanctions on the authoritarian government.
Administration officials consider the diplomatic opening to the long-isolated nation, also known as Burma, a marquee achievement in President Obama's first term.