WASHINGTON >> President Barack Obama could lift restrictions on arms sales when he makes his first visit to Vietnam next week. That would remove a final vestige of wartime animosity but would not please China, which views growing U.S. defense ties in its backyard with deep suspicion amid rising military tensions in the South China Sea.

There’s considerable support in Washington for the lifting the restrictions, including from the Pentagon, but also pockets of congressional opposition, leaving uncertain whether Obama will announce it when he visits Vietnam, starting Sunday. The administration is pushing for more progress on human rights, a constant drag on the relationship. Significantly, the communist government has committed to allow independent labor unions as a condition of its participation in the U.S.-backed Trans-Pacific Partnership trade deal, but it still holds about 100 political prisoners and there have more detentions this year.

As part of Obama’s effort to help Southeast Asian nations counter Beijing, the U.S. in 2014 partially lifted an arms embargo in place since the end of the Vietnam War, allowing Vietnam to buy lethal defense equipment for maritime security. Vietnam, which has mostly Russian-origin equipment, has not bought anything, but is still eager for Washington to remove the remaining restrictions. If nothing else, it would show relations are fully normalized and open the way to deeper security cooperation.

“Real progresson protecting human rights and fundamental freedoms including through legal reform is crucial to ensuring that Vietnam and our relationship achieves its full potential,” Daniel Kritenbrink, the White house senior director for Asian affairs, told reporters Wednesday. He refused to comment directly on whether the remaining arms sale restrictions would be lifted — also sensitive because of criticism of Vietnam’s rights record among congressional opponents of TPP.

The risk of confrontation with Beijing is already growing as the U.S. challenges China’s island-building and assertive behavior in the South China Sea, where five other Asian governments, including Vietnam, have territorial claims. The Pentagon said that two Chinese fighter jets flew Tuesday within about 15 meters (50 feet) of a U.S. Navy reconnaissance plane, forcing the pilot to descend sharply to avoid a collision. China on Thursday denied its behavior was unsafe, and demanded the U.S. stop spying.

China would view the lifting of the restrictions as an attempt to woo Vietnam closer to the U.S. and away from China. “It will undoubtedly be seen as aimed at weakening China’s position and influence in the region,” said Bonnie Glaser, a China expert at the Center for Strategic and International Studies, or CSIS.

But Beijing will be guarded in its reaction because Vietnam is a fraternal communist neighbor. Asked about the prospect of the U.S. lifting arms restrictions, Foreign Ministry spokesman Hong Lei said Thursday that China “hopes the countries concerned will play a constructive role in ensuring their cooperation be conducive to the regional stability and safety.”

Hanoi and Beijing have an ambivalent relationship. Despite the ties between their ruling parties, they fought a border war in 1979 in which thousands died, and clashes in 1988 over their conflicting claims in the South China Sea claimed dozens of lives. Those tensions reared again in 2014, when China parked an oil rig off Vietnam’s central coast, sparking confrontations at sea and deadly anti-China riots in Vietnam.

“The Vietnamese have got a very tough strategic equation to solve,” said Marvin Ott, a former National War College lecturer who led the first, cautious military-to-military contacts between the U.S. and Vietnam in the mid-1990s. One aspect is how far Vietnam can go in deepening relations with the U.S. without provoking China. The other is placating U.S. demands for progress on democracy and human rights without threatening the ruling party’s grip on power, he said.

Obama will be the third consecutive U.S. president to visit Vietnam since diplomatic relations resumed in 1995. In 2013, the two sides declared a comprehensive partnership, and last July, the chief of Vietnam’s Communist Party visited the White House, showing that resistance among party hardliners to deeper ties with Washington was receding.

But anxiety about China and memories of the Vietnam War still limit military cooperation, said Murray Hiebert, a CSIS expert on Southeast Asia. Despite Vietnam’s desire for the U.S. to lift restrictions and its interest in modernizing its defense equipment, buying from Russia is cheaper and easier.

According to Stockholm International Peace Research Institute, Vietnam has been the world’s eighth largest importer of weapons over the past five years.

Ott said that among South China Sea coastal nations, Vietnam is potentially the most significant military partner for the U.S. Among the others, Indonesia says it has no territorial dispute with China although they have overlapping maritime claims; the military of the Philippines, a U.S. ally, is weak; and Malaysia and Brunei are unwilling to confront China.

“If you’re sitting in the Pentagon, there’s only one country that actually could be a military partner and a factor in the South China Sea, and that’s Vietnam,” Ott said.

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Associated Press writer Nancy Benac in Washington, and writer Christopher Bodeen and researcher Yu Bing in Beijing contributed to this report.