Vietnam and the United States will stage a fresh demonstration this week of their military ties at a time of escalating tensions with China over territorial claims in the South China Sea.

The US destroyer USS John S. McCain, making a call at the central port city of Danang with a crew of around 270 sailors, departs Tuesday for a four-day exchange programme with the Vietnamese navy.

"On the 12th we will have the first ever training exchanges with the Vietnamese navy on damage control, emergency repair and fire fighting," said Mike Morley, the ship's public affairs officer.

The port call will take in sports events and medical and dental projects, according to a statement sent by the American embassy in Hanoi.

A mine countermeasures ship, the USS Avenger, was supposed to make a port call in Danang along with the USS John S. McCain but it had to stay in Japan for "operational reasons", Morley added.

On Sunday, the aircraft carrier USS George Washington, which recently took part in joint military exercises with South Korea which were denounced by Beijing, already hosted a delegation of Vietnamese military in the waters of the South China Sea off Danang.

The visit by the US Navy vessels, part of official celebrations marking the 15th anniversary of the normalisation of diplomatic relations between the former enemies, has taken on greater significance due to recent regional friction.

"Tensions in relations between China and the US are much more tense than a year ago and this has spilled over to the South China Sea," said Carl Thayer, a Vietnam expert at the Australian Defence Force Academy in Canberra.

"The US is demonstrating a military presence and Vietnam is letting this occasion speak for itself," he said.

Hanoi normally treads carefully in its relations with its Chinese ideological ally, but the regional ambitions of its large neighbour have stoked a degree of apprehension in Vietnam.

Relations between Hanoi and Washington have come a long way since the end of the Vietnam War in 1975 and the normalisation of diplomatic relations between the two 20 years later.

The United States has become an important partner of Vietnam, and not only in the commercial sphere.

Lately Hanoi has become particularly "uneasy about Chinese territorial ambitions", a Vietnamese defence ministry source said.

"The presence of American ships in Vietnamese ports has a great strategic significance for Vietnam," which "shows a regional balance in defence matters", he said.

"I think that the United States could play a much more important role in the region," he added.

The South China Sea has long been the subject of territorial disputes, particularly over the Paracel and Spratly archipelagos.

Since 1974, Beijing has been in de facto occupation of the Paracels, which are also claimed by Hanoi. China also claims sovereignty over the Spratlys, as does Vietnam, Brunei, Malaysia, the Philippines and Taiwan.

Vietnamese foreign ministry spokeswoman Nguyen Phuong Nga last week denounced China for sending ships to carry out seismic studies in the Paracels zone, which "violated Vietnam's indisputable sovereignty".

Top US commanders have made it clear they are keeping a close eye on Beijing's military build-up, in particular its naval presence in the South China Sea.

China last month held a large naval and air exercise in the South China Sea, a week after US Secretary of State Hillary Clinton said resolution of the territorial disputes was "pivotal" to regional stability.

Speaking at Asian security talks in Hanoi, Clinton indicated that Washington was prepared to facilitate multilateral discussions on the island chains and said the US had a "national interest" in seeing international law respected in the area.

Beijing swiftly denounced any such attempts to "internationalise" the disputes, saying such moves could worsen the situation.

And visits by US naval ships to Vietnam risk further irritating China.

"The Vietnamese are ever sensitive to how China will react," said Thayer. "The visit of US warships to Danang indicates that Vietnam is willing to risk Chinese verbal displeasure."