Biden -- who is flying from the G20 summit in New Delhi -- will meet the leader of Vietnam's ruling Communist Party, Nguyen Phu Trong, on Sunday to sign off on a "comprehensive strategic partnership", Hanoi's highest level of diplomatic ties.

The underlying goal of the short visit will be similar to Biden's time at the G20 gathering -- to shore up support against China's increasing influence.

For Vietnam, the upgrading of diplomatic ties is significant. It only has top-level ties with Russia, India, South Korea and China.

Although it will be careful to be seen as not taking sides between the United States and China, Vietnam shares American concerns about its neighbour's growing assertiveness in the contested South China Sea.

However, the New York Times reported just ahead of Biden's visit that Vietnam was secretly arranging to buy arms from Russia in contravention of US sanctions.

The report cited a Vietnamese finance ministry document that laid out plans to finance arms purchases from the Kremlin through a joint oil and gas project in Siberia.

AFP has contacted the Vietnamese government for comment.

Jon Finer, the US deputy national security advisor, told reporters Sunday that Washington acknowledged Vietnam's decades-long military relationship with Russia.

But he said there was "increasing discomfort on the part of the Vietnamese with that relationship", and the United States and allies could help Hanoi "diversify away from those partnerships".

The United States and Vietnam -- a key manufacturing hub -- also have increasingly close trade ties, and Washington sees Hanoi as an important partner as it looks to source less from China after supply chain shocks rocked the global economy in recent years.

The two sides will likely announce new cooperation on semiconductors, Finer said.

There will be a welcome ceremony in Hanoi on Sunday, speeches by the two leaders and a news conference by Biden -- who on Tuesday awarded the top US military honour to a helicopter pilot who rescued four soldiers during the Vietnam War.

Biden will meet President Vo Van Thuong and Prime Minister Pham Minh Chinh on Monday.

Hanoi's central Hoan Kiem Lake area, packed with families out for a weekend stroll, was adorned with American and Vietnamese flags ahead of the 80-year-old US president's arrival.

A souvenir shop nearby in the city's old quarter sold T-shirts with Biden's face emblazoned across the front.

"I think the US is a good friend to Vietnam," said Truong Thanh Duc, the shop's 61-year-old owner.

"With this visit of President Joe Biden, I think he will bring more business contracts and jobs to Vietnamese people."

- Human rights -

Biden will be juggling strategic interests with the defence of human rights in Vietnam.

The Southeast Asian country has a dire human rights record. Government critics face intimidation, harassment and imprisonment after unfair trials, and there are reports of police torture to extract confessions, Human Rights Watch says.

While Biden has often criticised China's human rights record, he has largely stayed quiet on Vietnam and campaigners are fearful he may not press the subject.

National Security Advisor Jake Sullivan said before the trip that Biden would raise issues related "to freedom of expression, freedom of religion, and other basic human rights".

Biden's visit comes days after a US government commission on religious freedom harshly criticised Vietnam for "egregious, ongoing, and systematic violations".

Nguyen Bac Truyen, a legal expert and religious freedom advocate who was sentenced in 2018 to 11 years in prison for subversion, said on Facebook on Saturday he had been released and allowed to travel to Germany with his wife.

Vietnam often releases political prisoners before US presidential visits.

Leaders at the G20 summit in India agreed on a joint declaration that papered over deep divisions on the war in Ukraine and tackling climate change, avoiding direct criticism of Moscow and any concrete pledge to phase out polluting fossil fuels.

Biden's Vietnam trip will also include a visit to the memorial to his friend John McCain, the former US senator shot down and held captive during the Vietnam War who in later years helped rebuild ties between the two countries.

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