With increasingly few of their families left alive and acidic soil eating into their buried remains, time is running out to find the last Americans listed as missing in action from the Vietnam War.

But for the head of a relatives’ group, giving up is not an option.

"We need to act now," said Ann Mills-Griffiths, executive director of the National League of POW/MIA Families, whose brother has been missing since September 1966 when his navy aircraft disappeared over north Vietnam.

Almost four decades after the end of US combat involvement in the Southeast Asian nation, few parents of about 1,700 missing soldiers and airmen are still alive and siblings are often in their 60s.

Witnesses are also ageing, their fading physical and mental abilities limiting their ability to assist in investigations.

Mills-Griffiths, 69, was in Hanoi for talks that ended at the weekend with Vietnamese officials about a "renewed effort" to open up archival records which could provide vital clues as to what happened in some of the cases.

She was optimistic that the records would be made available, with "very good commitment" from the Vietnamese during the talks.

"But we’re running out of time," warned Mills-Griffiths, who has made about 30 trips to Vietnam as head of the families’ group.

The US and Vietnam have cooperated on investigations into missing American servicemen since 1985 – "the bridge," she says, which led to a normalization of diplomatic relations 10 years later.

While Vietnamese authorities have been very helpful in individual cases, Mills-Griffiths said the US would like broader access to archives related to areas along the Laotian border "where we have many, many losses."

"We know their record-keeping was almost of a scientific nature," said Richard Childress, a Vietnam veteran who is senior policy adviser to Mills-Griffiths.

Investigations are handled by the Hawaii-based Joint POW/MIA Accounting Command (JPAC), which conducts research, witness interviews and field analysis at sites in Vietnam where remains of missing servicemen are suspected to be.

The Americans work alongside a Vietnamese liaison team and when they have enough evidence, separate JPAC crews are called in to dig and try to find the remains which is where time is also against them.

"There’s a lot of acidic soil in this area. In fact, it has eaten away at the remains and there are smaller and smaller pieces to recover," said Mills-Griffiths.

At the same time, scientific advances mean that, particularly with dental remains, there is a very high chance of individual identification, she told AFP.

Hanoi says about 300,000 of its own soldiers are still listed as missing from the war. Of the American MIAs (Missing in Action), about 1,300 cases are in Vietnam. Most of the rest, in Cambodia and Laos, were in areas under Vietnamese control, Mills-Griffiths said.

Two years ago Hanoi proposed stepping up the pace and scope of searches but budget delays in US Congress have limited the US ability to respond, Mills-Griffiths said.

If eventually approved, the fiscal year 2012 budget includes "a significant increase" for JPAC that would allow it to deploy more teams, she said.

This year JPAC plans four month-long missions for investigation and excavation, with one under way now.

The remains of some servicemen will never be found but Mills-Griffiths said her league seeks the "fullest possible" accounting, so family members know all efforts have been made to clarify the fate of loved ones.

Mills-Griffiths estimates the US has spent "countless millions" of dollars on the effort.

"I’m sure some people might think that it’s like an exercise in looking at the past," she said, but "if I become captive or missing I want to know somebody is there to return me to my family and to the country that I served."

Mills-Griffiths said that because of her position she has never discussed the case of her brother, Lieutenant Commander James Mills, with the Vietnamese.

"What has kept me involved all this time is not the fact that I have a brother who’s missing. What’s kept me involved is the principles of it, and knowing it’s the right thing to do, for a nation to do."