YANGON, Myanmar — Secretary of State Hillary Rodham Clinton said on Thursday that the United States would loosen some restrictions on international financial assistance and development programs in Myanmar, in response to a nascent political and economic opening in the country.

The United States and Myanmar, formerly known as Burma, also agreed to discuss upgrading diplomatic relations — which were suspended for two decades — and exchanging ambassadors, a step that could transform American diplomacy in Southeast Asia.

Mrs. Clinton met the country’s new president, U Thein Sein, on Thursday morning and its main opposition leader, the Nobel peace laureate Daw Aung San Suu Kyi, later in the day, underscoring the Obama administration’s cautious efforts to nurture a thaw in one of the world’s most isolated and repressive nations. In each meeting, Mrs. Clinton delivered a letter from President Obama, expressing support for the democratization of Myanmar.

“For decades, the choices of this country’s leaders kept it apart from the global economy and the community of nations,” Mrs. Clinton said after meeting Mr. Thein Sein in Naypyidaw, Myanmar’s remote new capital. “Today, the United States is prepared to respond to reforms with measured steps to lessen its isolation and improve the lives of citizens.”