Former general Thein Sein on Monday becomes the first Myanmar president to be welcomed to the White House in almost 47 years, crowning a dramatic diplomatic rehabilitation for his nation after years of international isolation.

But activists are angry about President Barack Obama hosting Mr. Thein Sein, and lawmakers are wary. The Myanmar leader has led the shift from decades of direct military rule, but has stalled on some reform commitments and failed to stop bloody outbursts of ethnic violence.

Mr. Thein Sein previously served in a repressive junta, and his meetings at the White House and Congress would have been all but impossible, before he took the helm of a nominally civilian government in 2011. His name was only deleted from a blacklist barring travel to the U.S. last September.

Myanmar has been rewarded by relaxation of tough economic sanctions, and Mr. Sein will be addressing the U.S. businessmen, keen to capitalize on the opening of one of Asia’s few untapped markets.

“President Thein Sein’s visit underscores President Obama’s commitment to supporting and assisting those governments that make the important decision to embrace reform,” the White House said in its announcement of Monday’s visit.

It will be the first by a Myanmar leader since a September 1966 visit by Ne Win, an independence hero-turned dictator, who began the nation’s descent from regional rice bowl to economic basket case.

Mr. Sein visited New York last September for the U.N. General Assembly, but did not come to Washington.

The U.S. last month announced it is considering duty-free access for Myanmar to U.S. markets, and there could be progress on Monday towards a bilateral trade and investment framework agreement.

The most significant outcome of Mr. Sein’s trip could be a symbolic one. Obama is expected to use “Myanmar” the country name adopted by the junta in 1989 when he meets Mr. Sein. However, the U.S. will keep using “Burma” in official documents.

Mr. Sein will be accorded the protocol due to a foreign president, yet his Washington welcome will pale next to that granted last September to Nobel laureate Aung San Suu Kyi, the opposition leader who met Obama and was presented by Congress with the highest civilian award it can bestow.

Ahead of the trip, Myanmar released at least 19 political prisoners in what has become a pattern for amnesties that coincide with high profile international meetings as a way of highlighting the government’s benevolent policies. Right groups say at least 160 political detainees are still held.

Mr. Sein’s office director, Zaw Htay denied in a posting on his Facebook page on Friday that the government was using political prisoners as “tools,” saying the president was striving for an “all-inclusive political process.”

There has been mixed progress on 11 reform commitments made in November just before President Obama visited.

The government has permitted the International Committee of the Red Cross, access to its notorious prisons for the first time in seven years. But hasn’t allowed adequate humanitarian access to conflict zones where tens of thousands have been displaced. Authorities have failed to stop, and may have abetted in some cases, an explosion in communal violence that has killed hundreds and led to segregation of Muslim communities.

The U.S. Campaign for Burma said Mr. Sein’s trip follows a troubling downward trend in Myanmar, and that “instead of honoring an abusive leader” the U.S. should tie its concessions to conditions.

The Obama administration’s engagement with Myanmar has been a rare area of bipartisan agreement in Washington, but some lawmakers have begun to voice concern that the U.S. could be moving too fast. House members have opposed moves for resumption of even preliminary military-to-military cooperation.

“I’m incredibly concerned about the facts on the ground in Burma, including human rights violations against ethnic nationalities, the use of rape as a weapon of war and brutal violence against Muslims including women and children,” Democratic Rep. Joe Crowley, a prominent voice in Congress on Myanmar, said in a statement. He urged the administration to stick to its initial policy of “action for action” in its relations with Myanmar.

On the eve of Mr. Sein’s arrival, Crowley and Republican Rep. Peter King introduced legislation aimed at extending a ban on gems imports from Myanmar that will lapse in July. Much of the jade and rubies Myanmar exports come from northern Kachin State, scene of bitter fighting in recent months between the army and ethnic rebels.