Hanoi and Washington have improved defence links in recent years as they seek to counter Beijing's growing might in the disputed and resource-rich waterway.

The US Coast Guard gave its Vietnamese counterpart six fast-response Metal Shark boats, which "will serve to deter bad actors from committing crimes against or near Vietnam", the embassy said in a statement, citing among other threats environmental degradation, illegal fishing and piracy.

The delivery, part of a deal announced in 2016, took place during a visit to the country by the US Coast Guard's Pacific Area commander Vice Admiral Fred M. Midgette.

It comes only three weeks after the historic visit of US aircraft carrier USS Carl Vinson, the first since the end of the Vietnam war in 1975.

The boat handover is the second of its kind and part of a US package of $20 million worth of equipment for the Vietnamese Coast Guard station on Phu Quoc island in the southwest.

The move demonstrates US support for a "strong, prosperous, and independent Vietnam that contributes to international security and the rule of law", the embassy statement added.

Tensions in the South China Sea are ongoing as China continues to build artificial islands capable of hosting military installations.

Vietnam, along with several other Southeast Asian nations and Taiwan also claim parts of the waterway, which is believed to sit atop vast oil and gas deposits and through which $5 trillion in trade passes annually.

Vietnam remains the most vocal in the South China Sea dispute with the Philippines backing off under China-friendly President Rodrigo Duterte.

Security ties between the US and Vietnam have deepened since Washington normalized relations with Hanoi in 1995.

Last August, Vietnam's Defence Minister Ngo Xuan Lich visited the US, part of Hanoi's campaign to keep Washington close under US President Donald Trump.

In January, US Defence Secretary James Mattis made a two-day visit, asserting that freedom of navigation in the sea was crucial for the fast-growing Southeast Asian country.