Key Points:

CANBERRA - The United States planned to gas Australian troops in experiments with two of the most lethal nerve gases ever devised, newly declassified files have revealed.

Previously top secret documents have shown that even as the world was outlawing chemical weapons at the height of the Cold War, Washington sought Canberra's permission to test sarin and VX gas on diggers in remote Queensland.

The documents, shown on Channel Nine's Sunday programme yesterday, indicate that US military scientists wanted to bomb and spray 200 "mainly Australian" troops with the deadly nerve agents in the 1960s.

Shaken by the request, the plan was rejected by Liberal Prime Minister Harold Holt, despite Canberra's deep concern to keep the US engaged in the western Pacific.

At the time the two nations were embroiled in Vietnam and in contingency plans for war against China drawn up by the former Southeast Asia Treaty Organisation.

Sarin and VX gas are now classified as weapons of mass destruction and are outlawed under the 1993 Chemical Weapons Convention.

First produced by Nazi Germany but not used in World War II, Sarin is fatal at even very low concentrations, killing within minutes.

Produced by both the US and the former Soviet Union, the gas was adopted as a standard chemical weapon in Europe by the North Atlantic Treaty Organisation but was later dumped from Western arsenals.

But Sarin was used by Iraq during its eight-year war with Iran in the 1980s, and was used by late dictator Saddam Hussein in an attack against the Kurdish city of Halabja, killing an estimated 5000 people.

The other nerve gas named in the US proposals for Australian tests, VX, is regarded as the most toxic nerve agent ever produced, and was manufactured in large volumes by America during the early 1960s.

Channel Nine reported that Washington had asked Holt to allow testing of the two gases against Australian troops, probably in the Iron Range rainforest near Lockhart River in far north Queensland.

It said planning was very advanced in the US, which wanted the operation to be kept secret because the weapons were illegal under international law.

Survey teams inspected the proposed testing site, and details of the tests were to be kept secret from all but a handful of troops.

Peter Bailey, a former senior adviser to Holt, told the programme that the request alarmed Canberra. "If they weren't pretty good and pretty faithful to the Americans we would be dumped, so I think ministers were very aware that this was probably our one main support," he said.

Former Democrats Senator Lyn Allison told Channel Nine the present Labor Government should make the documents public.

And toxicology Professor Chris Winder of the University of New South Wales said the idea that Australia could even countenance such tests was unacceptable. He said Cold War fears that Chinese or Russian attackers might have used such weapons in a third world war "doesn't justify it now and I don't think it justified it then".