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

In a decision likely to anger China, the US is partly lifting a 40-year ban on arms sales to former foe Vietnam to help boost 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 official, 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 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.

Kerry later praised "the transformation" in Vietnam since the US normalized diplomatic relations two decades ago, calling it "nothing short of amazing."