In a remote valley in Vietnam, US investigators sift through piles of red soil. Despite recovering the remains of hundreds of fallen troops, the hunt goes on for many more still missing in a race against time.

With witnesses ageing, acidic soil eating into remains, and rapid development encroaching on areas where troops died during the Vietnam War, investigators warn there is little time left before all evidence is lost.

In May 1968, explosions shook the now-quiet valley in a battle around the Kham Duc Special Forces Camp in Quang Nam province -- an hour-long helicopter ride over green, terraced paddy fields from Danang airport, a former US base.

More than 40 years later a joint US-Vietnamese recovery team is hunting for the remains of those who were lost -- and must find them before all traces disappear.

"We most often find bits of metal fragments -- the area was heavily bombed. About 90 percent of what gets found (bone fragments and other remains) comes through the sieves," the team's anthropologist, Mindy Simonson, told AFP.

In Kham Duc, Vietnamese workers in a long line pass buckets of earth from a shallow pit neatly marked out with twine to the "evidence station" where it is sifted through dozens of large sieves.

Nearly 60,000 American soldiers died in the bloody Cold War-era conflict, which also claimed the lives of up to three million Vietnamese civilians and soldiers before ending in 1975 with Vietnam's reunification.

When the guns fell silent, 1,971 Americans were left unaccounted for, according to figures from the US Joint Prisoners of War, Missing in Action Accounting Command (JPAC), which handles the search for MIAs.

Since then, 687 have been identified and repatriated and 586 are listed as "no further pursuit", meaning it is not possible to recover their remains. But JPAC is still hunting for about 700 missing individuals.

"It is no secret that the remains being recovered from most of the sites in Vietnam these days are fragmented and small," said JPAC's Ron Ward. "Remains are being lost to acidic soil, scavenging and land development."

The search is now a "race against time", he told AFP, but added there is "light at the end of the tunnel"

"We are relatively close to completing the majority of the excavations," Ward said.

At the Kham Duc dig, the team was guided by a younger, local witness who had stumbled upon the site while scavenging for scrap metal. "Witness" is the term used for people who know where the sites are.

Usually witnesses are elderly -- another factor JPAC is "racing against," said Lieutenant Colonel Patrick Keane. In many cases it is former soldiers who come forward with information.

"The Vietnamese witnesses are getting exceptionally old and we're losing those first hand witnesses. Once you lose a first hand witness, your ability to actually close a case... pretty much comes to an end," Keane told AFP.

Moreover, as Vietnam develops, more and more land is being turned into roads, hotels or coffee plantations.

"The third and fourth quarter of last year, every site we went to was in imminent danger" due to development, said Keane, the head of JPAC's operations in Vietnam.

The fact that remains are so fragmented and being eaten away by acidic soil makes it harder for JPAC scientists to do a positive DNA identification -- essential if a case is to be closed, he said.

When pressed on how long it would be until evidence disappeared, Keane said it was impossible to talk about exact timescales as there were many variables.

"There's no way to say 'well, in 6.2 years all the bones will be gone'. It's just we understand it is a concern, and it is something that we're racing against to ensure that we do find enough remains for the laboratory to be able to execute some sort of scientific (analysis)."

"The other factor that is important to us is we have mothers, fathers, brothers, sisters, and children who are all waiting for word on their loved ones, and they're all getting older," he added.

Unidentified remains found on JPAC digs are placed in flag-draped coffins and flown to JPAC's labs in Hawaii where they are DNA-tested -- families are needed to provide samples.

After they have been identified, which can take some time depending on how quickly the DNA sequencing can obtain a positive identification, remains are flown to the mainland and returned to families for proper funerals.

According to JPAC's Ward, "the best-case scenario would be to reach a fullest possible accounting of (MIAs) within the lives of the immediate family members of the missing. There are very few parents left," he told AFP.

Bilateral cooperation has improved dramatically since the early days and the use of Vietnamese Recovery Teams -- local teams with a small American presence -- have speeded up the search.

"We want to pick up the pace so we can finish in time," said Colonel Dao Xuan Kinh, 61, who oversees the Vietnamese side of the dig effort for US remains.

The MIA issue was one of the first points of contact between the former enemies, which helped pave the way for the resumption of ties in 1995.

Vietnam also wants US help with finding its own missing soldiers, Kinh said.

Hanoi says about 300,000 North Vietnamese soldiers are still listed as missing from the war. The number of South Vietnamese MIAs remains unclear.

A key factor constraining the speed of the mission is cost -- JPAC estimates their Vietnam operation's annual operating budget, excluding field operations, at about $15 million.

This year, a funding boost has enabled the operation to field more recovery teams.

This has, Keane estimates, cut the estimated end date from 20 years to seven years -- if work continues at the current pace.

The Veterans of Foreign Wars of the United States (VFW) organisation says the hunt must continue whatever the cost.

"As long as there is one Vietnam veteran alive, this mission will continue," said VFW's Adjutant General Allen F. Kent.

"The American soldier was always told -- you go into combat and you defend your country and your country will make sure you come home properly to be buried on the ground that you were born in."