WASHINGTON — The Obama administration on Wednesday announced its most significant moves yet to open relations with Myanmar, lifting the travel ban on its senior leaders and easing some sanctions that have starved the country of most American investments for more than two decades.

Only days after special elections that brought the country’s Nobel Prize-winning dissident, Daw Aung San Suu Kyi, and her party into office for the first time, the administration also said it would reopen a United States Agency for International Development office, clearing the way for an expansion of foreign assistance. It also will remove restrictions on the work of American nongovernmental organizations in areas like health, education and the environment.

The administration’s actions do not lift the sweeping array of economic sanctions in place against Myanmar, formerly known as Burma, but the announcement punctuated an extraordinary and swift warming of relations between two countries that have been estranged ever since Myanmar’s military junta threw out the results of democratic elections in 1990 won by Ms. Aung San Suu Kyi’s party.