Vietnam's president is starting a rare visit to Washington to boost trade and security ties between the former war foes, but activists urged the United States to press him on human rights.
President Truong Tan Sang becomes only the second Vietnamese head of state to visit the White House since the countries normalised ties.
He was due to meet business leaders and Secretary of State John Kerry.
US officials said Barack Obama hoped to work with Sang on trade, as Vietnam is one of a dozen nations negotiating the ambitious TransPacific Partnership, and on global efforts to fight climate change.
The two nations have steadily boosted military co-operation, with Vietnam worried over what it sees as assertive claims to disputed territories by China.
Obama has put a priority on Southeast Asia, seeing the economically growing and mostly US-friendly region as neglected in the past. Sang will be the fourth Southeast Asian leader at the White House this year.
Danny Russel, the top US diplomat for East Asia, called Southeast Asia "perhaps our most vibrant, our most dynamic region" in Asia or even the world, hailing Sang's visit as "quite a historic milestone".
But US lawmakers and activist groups have demanded that Obama put a focus on human rights, accusing him of sending mixed signals by welcoming Sang at a time that even US officials say Vietnam has stepped up repression of dissent.
State Department officials testified to Congress last month that Vietnam was holding more than 120 political prisoners and increasing restrictions in several areas, notably cracking down on internet freedom.
In a joint letter to Obama, family members of some 35 detained Vietnamese activists or bloggers urged the US leader to "stand up for the people of Vietnam" by pressing Sang to free all political prisoners.
The family members drew a parallel to Myanmar, which has undertaken several years of democratic reforms.
The relatives noted that Obama invited Myanmar's President Thein Sein only after the release of a significant number of political prisoners.
US lawmakers have warned that they may fight the Trans-Pacific Partnership - which Obama has billed a potential landmark agreement in shaping Asia's new order - unless Vietnam commits to human rights improvements.
Sang, who is stopping in New York as well, visited Hawaii last year for an Asia-Pacific summit.