The US and Vietnam are conducting joint naval exercises in the South China Sea, a sign of increasing military ties between the two former enemies.
The week-long activities focus mainly on non-combatant exercises and are part of the 15th anniversary of diplomatic ties between Washington and Hanoi.
But correspondents say they could anger China, which has been in a dispute with Vietnam over islands in the area.
Tensions over territories in the South China Sea have increased recently.
The US has described the exercises as a "series of naval engagement activities" which focus mainly on damage control and search and rescue.
The US Navy on Sunday hosted Vietnamese military and government officials on the USS George Washington, which is on its way back from naval exercises with South Korea in the Sea of Japan.
Its strike group of three destroyers are also in the South China Sea, while the USS John S McCain is due to call at the Vietnamese port of Danang later on Tuesday.
'Belong to nobody'
Nga Pham of the BBC's Vietnamese service says the training shows a remarkable warming of the relationship between the US and Vietnam.
It also shows Washington's determination to defend free navigation in the area contested by a number of south-east Asian countries and China, our correspondent adds.
The US has increased its presence in the region in recent months as part of a show of support for South Korea against North Korea, which has been blamed for the sinking of the South Korean vessel Cheonan in March.
Capt David Lausman, commanding officer of the George Washington, said on Sunday that the waters "belong to nobody, yet belong to everybody".
"China has a right to operate here, as do we and as do every other country of the world," he added.
Last week, the Vietnamese foreign ministry said that Chinese ships which were carrying out seismic studies in the Paracels zone - a disputed area - had breached Vietnam's sovereignty.
Beijing has always protested against the involvement of any other parties in the South China Sea dispute, saying it was a matter for "bilateral negotiation".