WASHINGTON — The White House formally eased sanctions on Myanmar on Wednesday, signaling support for its recent government reforms and officially opening the door to American investment in the deeply impoverished Asian nation.

Secretary of State Hillary Rodham Clinton had announced plans in the spring for a ‘‘targeted easing’’ of sanctions to allow American dollars to enter the country, for decades one of the most authoritarian in the world.

The United States still will not allow investment in entities owned by Myanmar’s armed forces or Ministry of Defense. It also bolstered its ability to sanction ‘‘those who undermine the reform process, engage in human rights abuses, contribute to ethnic conflict, or participate in military trade with North Korea.’’

“Individuals who continue to engage in abusive, corrupt, or destabilizing behavior going forward will not reap the rewards of reform,’’ President Obama said in a statement, while praising the country’s progress.

The announcement came on the day that Derek J. Mitchell, the first US ambassador to the former Burma since 1990, arrived at his posting.