HANOI: US Defence Secretary Lloyd Austin sought on Thursday to nudge forward security ties with Vietnam that have been slowly deepening as both countries watch China's activities in the South China Sea with growing alarm.

Despite closer military relations, more than four decades after the Vietnam War ended in 1975, President Joe Biden's administration has said there are limits to the relationship until Hanoi makes progress on human rights.

Vietnam has emerged as the most vocal opponent of China's territorial claims in the South China Sea and has received US military hardware, including coastguard cutters.

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Before a meeting with his Vietnamese counterpart in Hanoi, Austin said the United States did not ask Vietnam to choose between countries.

"One of our central goals is ensuring that our allies and partners have the freedom and the space to chart their own futures," Austin said.

He did not mention China but there is a perception in Asia that China is making countries chose between it and the United States, as tension rises between those two big powers.

On Wednesday, a US Navy warship carried out a transit through the Taiwan Strait. While such operations are routine, they usually anger Beijing.

"(Vietnam) wants to know that the US is going to remain engaged militarily, it's going to continue its presence in the South China Sea," said Greg Poling, with the Center for Strategic and International Studies.

The two sides signed a "memorandum of understanding" for Harvard and Texas Tech University to create a database that would help Vietnamese search for those missing from the war.

LIMITS

On Sunday, the United States shipped 3 million doses of the Moderna COVID-19 vaccine to Vietnam, raising the amount given by the United States, via the global COVAX vaccine scheme, to 5 million doses.

Poling said there was a limit to how fast and far the Vietnamese were comfortable with deepening ties.

Experts say there are lingering concerns in Vietnam about Biden's predecessor, Donald Trump, withdrawing from the Trans Pacific Partnership trade pact in 2017.

There are also limits to how far the United States is willing to deepen relations before Vietnam improves its human rights record.

Vietnam has undergone sweeping economic reforms and social change in recent decades, but the ruling Communist Party retains a tight grip over media and tolerates little dissent.

In Singapore on Tuesday, Austin said the United States would always lead with its values.

"We will discuss those values with our friends and allies everywhere we go and we don't make any bones about that," Austin said.

This month, Marc Knapper, Biden's nominee to be the next US ambassador to Vietnam vowed to boost security ties but said they could only reach their full potential if Hanoi made significant progress on human rights.

In a meeting with Austin on Thursday morning, Vietnamese President Nguyen Xuan Phuc said he was looking forward to an upcoming visit to Vietnam by US Vice President Kamala Harris.

Harris could travel to Vietnam and Singapore in August, a source familiar with the matter told Reuters on Monday.