The US has partially lifted a long-time ban on lethal weapon sales to Vietnam to help it improve maritime security, a move that comes nearly 40 years after the end of the Vietnam War.

The easing of the ban in place since the end of the Vietnam War in 1975 will only apply to maritime equipment, State Department officials said, and comes amid warming ties and as Vietnam makes "modest" improvements to human rights.

"The State Department has taken steps to allow for the future transfer of maritime security-related defence articles to Vietnam," Jen Psaki, State Department spokesperson, said on Thursday.

State Department officials told a separate briefing that the decision would ease a ban on sales of lethal weapons to Vietnam that has been in place since the end of the Vietnam War, although only for maritime security purposes at this point.

US officials denied the policy change was an "anti-China" move and insisted they had no specific sales to outline so far, but would consider each request from Vietnam on a "case-by-case" basis.

The focus would be on helping Vietnam patrol and defend itself in the South China Sea, amid growing naval challenges from China, the officials said, but future weapons sales could include airborne systems as well as ships.

Rights group warning

US Senator John McCain, a former prisoner of war in Vietnam, welcomed the news but also called for more work on human rights.

"Easing the lethal arms ban on Vietnam for the purpose of maritime security will strengthen our defence cooperation in ways that benefit both countries," he said.

"How much more we can do in this regard, and how quickly we are able to do it, depends greatly on additional steps by Vietnam to respect and defend the human rights of the Vietnamese people."

The decision drew criticism from human rights groups worried that the US could lose leverage over Vietnam.

"It is too soon; they haven't earned it," John Sifton, Asia advocate at the non-profit Human Rights Watch, said.

"They are still arresting people. The number of arrests and convictions has gone down from its peak in 2013, but ... the raw number of people going into the system is still larger than the number of people being released."

Vietnam's foreign minister last week said his country would welcome an end to the arms embargo after Reuters reported that the US was nearing such a decision.

Source: Agencies