Vietnamese and US flags are seen flying atop a US ship in Tien Sa port, Vietnam in April 2012
Vietnam's president on Wednesday starts 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 will on Thursday become only the second Vietnamese head of state to visit the White House since the countries normalized ties. He will meet Wednesday with business leaders and Secretary of State John Kerry.
US officials said President Barack Obama hoped to work with Sang on trade, as Vietnam is one of a dozen nations negotiating the potentially ambitious Trans-Pacific Partnership, and on global efforts to fight climate change.
A generation after normalizing relations, the former enemies have steadily boosted military cooperation, with Vietnam worried over what it sees as assertive claims to disputed territories by historic adversary 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 only invited Myanmar's President Thein Sein after actions including the release of a significant number of political prisoners.
"An independent and democratic Vietnam will not only help advance the bilateral relations between the two countries, but will be a necessary condition that contributes to the stabilization and prosperity of the Asia Pacific region," they wrote.
The letter was signed by relatives of human rights attorney Le Quoc Quan, blogger Nguyen Van Hai and legal activist Cu Huy Ha Vu, among other detained Vietnamese.
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.
"A free trade agreement - specifically the Trans-Pacific Partnership -- must be met with benchmarks and we call upon President Obama to carry this message on Thursday," said Representative Loretta Sanchez, a member of Obama's Democratic Party.
Representative Ed Royce, a Republican who heads the House Foreign Affairs Committee, called Sang's visit "a unique opportunity to inspire the Vietnamese people who are yearning for freedom."
Royce and Sanchez both represent districts in southern California with large Vietnamese American communities, which tend to be critical of the communist leadership in Hanoi.
Protesters trailed the only previous Vietnamese president to visit the White House -- Nguyen Minh Triet, who was invited by then president George W. Bush -- during his six-day trip across the United States in 2007.
Sang, who is stopping in New York as well, visited Hawaii last year for an Asia-Pacific summit. Vietnamese prime ministers have also visited Washington several times since 2005.