Vietnam and the United States kicked off joint naval exercises on Friday in spite of disapproval from China amid tensions over sovereignty claims in the South China Sea.
The joint drills between the two former enemies were scheduled to last a week, and include a slew of activities like navigation and maintenance exercises, U.S. officials said.
The U.S. Consulate General in Ho Chi Minh City said in a statement that the events would comprise "noncombatant" activities.
Vietnam and China, as well as the Philippines, Brunei, Malaysia, and Taiwan, have overlapping claims to parts of the South China Sea, believed to be rich in oil and gas deposits and home to shipping lanes vital to global trade.
The Philippines and Vietnam have said they are alarmed by increasingly aggressive actions by China in the disputed waters, causing speculation that they would turn to Washington for support.
Earlier this week, Vietnam accused sailors from China's navy of beating the captain of a Vietnamese fishing boat and confiscating its catch.
The incident took place northeast of the potentially oil-rich Paracel Islands, according to the Vietnamese coast guard service.
China and Vietnam say they have sovereignty over the Paracels while another set of islands, the Spratlys, are also claimed by the two nations as well as by the Philippines, Taiwan, Brunei, and Malaysia.
China's top military officer Gen. Chen Bingde had said that the timing the U.S.-Vietnam joint naval exercises was "inappropriate."
Chen's comments came during talks with his U.S. counterpart, Admiral Mike Mullen, which were aimed at rebuilding trust following years of strained ties between the U.S. and Chinese militaries.
China and Vietnam have recently moved to calm tensions over the territorial dispute in the South China Sea, agreeing to hold talks and ruling out a military confrontation.
However, Vietnam has welcomed foreign involvement to resolve the rival claims.
American and Vietnamese officials have called for freedom of navigation in the South China Sea, which encompasses an area from the Singapore and Malacca straits to the Strait of Taiwan.
In June, Beijing and Hanoi agreed "to prevent words and actions that would be detrimental to the friendship and mutual trust between the peoples of the two countries," China's foreign ministry said.
Vietnam and China have held separate live-fire military exercises in the area after Hanoi accused Chinese ships of ramming one oil survey ship and cutting the exploration cables of another.
The standoff sparked a wave of anti-China protests in Vietnam and warnings from Beijing to Vietnam not to try to involve the United States.
Reported by Luisetta Mudie.