Joe Biden on Tuesday awarded the highest US military honor to an army helicopter pilot who saved four troops during the Vietnam War, in a ceremony days before the president heads to the Southeast Asian nation.

Larry Taylor, now 81, braved heavy enemy fire so a group of trapped soldiers could cling onto the skids of his two-seat Cobra helicopter, a never-before-attempted feat at the time.

"That's valor, that's our nation at its very best," Biden said before placing the Medal of Honor around the retired captain's neck in a ceremony in the East Room of the White House.

Taylor dabbed his eyes as Biden recounted the story of how the four members of a reconnaissance patrol were pinned down in a hamlet near Saigon on 18 June 1968.

His helicopter took several hits as it and another gunship made attack runs for 45 minutes to try to give the men an escape route.

But after learning that a rescue chopper had been called off, Taylor, then a first lieutenant, defied a "direct order" to return to base, said Biden. "His response was just as direct — I'm getting my men out."

He landed under heavy fire and the soldiers clambered on the skids and rocket pods of the helicopter, which lifted them to a safe location.

Taylor waited 55 years to receive the Medal of Honor, following a campaign by fellow veterans.

Biden, 80, whose wife Jill tested positive for COVID-19 on Monday, wore a black mask for the start of the ceremony but then removed it to speak.

He is set to visit Vietnam on Sunday for a meeting with the leader of the ruling Communist Party, as Washington seeks to counter China's influence in the region.

The Vietnam War ended in humiliation for the United States, with a traumatic evacuation from besieged Saigon in 1975, but the former foes have since fostered close ties.

White House National Security Advisor Jake Sullivan told a briefing earlier on Tuesday that the visit was part of a decades-long effort to "overcome a painful shared legacy of the Vietnam War."