The United States on Thursday renewed calls on Vietnam to improve its human rights record, saying it needed to do more to win support in Washington to expand cooperation.

Kurt Campbell, the top US diplomat for East Asia and advocate of warmer relations with the former war foe, said lawmakers have persistently voiced concern about Hanoi's treatment of dissidents, minorities and religious people.

"What has prevented the kind of rapid development in bilateral ties that some have hope for are continuing human rights issues inside the country," Campbell, an assistant secretary of state, told the Stimson Center think-tank.

Such concerns "have to be addressed more seriously to allow for the kind of support in the United States that we saw, for instance, when we established diplomatic relations" in 1995, Campbell said.

Rights groups say dozens of peaceful political critics and campaigners have been sentenced to long prison terms since Vietnam launched a crackdown in late 2009, despite Hanoi's stance that its rights record is improving.

Separately Thursday, the heads of a bipartisan US congressional committee on human rights jointly urged Vietnam to release prisoners including activist lawyer Cu Huy Ha Vu and dissident Catholic priest Nguyen Van Ly.

"There is no excuse for Vietnam to target its citizens who are exercising their political and religious rights in a peaceful manner," said Representative Jim McGovern, co-chairman of the committee named after late congressman Tom Lantos.

"The message is simple: Release all prisoners of conscience and allow the free expression of religion and thought," said McGovern, a member of President Barack Obama's Democratic Party from Massachusetts.

Vietnam has been seeking greater exchanges with the United States amid high tensions with China, particularly over disputes in the South China Sea.

"Vietnam is imminently aware of its position in Asia, its relationship with China, and I think our desire would be to have the kind of strategic interaction that would be transparent and open," Campbell said.

The United States has repeatedly pressed China to be more open about how it is spending its rapidly growing defense budget.