In a decision expected to anger China, the US is partly lifting a 40-year-old ban on arms sales to former foe Vietnam to help boost its defenses in the tense South China Sea.

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

"What's driving this is not a sudden desire to transfer military equipment to Vietnam writ large, but a specific need in the region," said one, highlighting what he called Vietnam's lack of capacity in the disputed waters and America's own national security interests.

"It's useful in trying to deal with the territorial disputes in the South China Sea to bolster the capacity of our friends in the region to maintain a maritime presence in some capacity."

Some 40 percent of the world's seaborne trade passes through the sea which is claimed in part by Vietnam, Taiwan, Brunei and Malaysia as well as China and the Philippines.

Although the United States has not taken sides in the territorial disputes, it has warned Beijing against "destabilizing actions" amid a series of tense maritime incidents.

Earlier this year Beijing placed an oil rig in waters also claimed by Vietnam, sparking deadly riots in the Southeast Asian nation.

Secretary of State John Kerry informed his Vietnamese counterpart Pham Binh Min during talks on Thursday of Washington’s move to adjust the current policy "to allow the transfer of defense equipment, including lethal defense equipment, for maritime security purposes only," a senior State Department official said.

The US ban on selling lethal weapons to Hanoi was being changed "because of our broader strategic interests in helping the countries of the region boost their ability to know what's going on in the maritime environment and to have a presence," he added.

- Not 'anti-China' -

However, a prohibition on sales of other kinds of lethal weapons would stay in place as Washington pushes Hanoi to improve its human rights situation.

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

And they sought to allay any concerns from Beijing saying it was purely a defensive measure.

"We're not talking about destabilizing systems, we're talking about defensive capabilities that would engender presence in the maritime spectrum. These are not things that are going to tip the regional balance," a second State Department official said, also asking not to be named.

Any sales would be done in close consultation with the US Congress, and would be heavily focused on equipping the Vietnamese coastguard, the State Department officials said.

So far, Washington has only been allowed to sell unarmed patrol boats to the Vietnamese coastguard since a total ban on military sales was lifted in 2006. That could now change, for example, the officials said.

And they acknowledged that airborne defense systems would also be considered for sale if they included a maritime capacity.

"This policy supports Vietnam's efforts to improve its maritime domain awareness and maritime security capabilities," State Department spokeswoman Jen Psaki told reporters.

Officials said however that the easing of the ban did not mean all arms sales were now on the table to the communist-run authorities amid continuing concerns about rights such as freedom of expression and religion.

"It's not an indication that we are going to provide all lethal assistance now, it's just simply says we can remove what heretofore has been a hinderance to our ability to provide legitimate maritime capacity," the second unnamed State Department official said.