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The illnesses suffered by veterans of the first Gulf War appeared to be linked to toxins including nerve gas, according to a US report.
The US Veterans Affairs Department said stress or mental illness did not explain most veterans' complaints, but there was a probable link to toxins.
British campaigners are demanding the government recognise "Gulf War Syndrome".
The UK's Ministry of Defence (MoD) says there is not enough evidence to prove its existence.
The report, by the Research Advisory Committee on Gulf War Veterans' Illnesses, said up to 30% of US Gulf War veterans had been afflicted by a "complex of multiple chronic symptoms over and above expected rates seen in veterans who did not serve in the Gulf War".
A growing body of research indicates that an important component of Gulf War veterans' illnesses is neurological in character.
It added: "Evidence supports a probable link between exposure to neurotoxins and the development of Gulf War veterans' illnesses."
It found veterans had developed Lou Gehrig's disease at about twice the rate of veterans who did not serve in the Gulf War.
Nerve agents Symptoms include headaches, memory problems, confusion, dizziness, blurred vision and tremors.
It said reports indicated a large number of Gulf War troops were exposed to a variety of potentially toxic substances, including low levels of chemical nerve agents, pills taken to protect veterans from the effects of nerve agents and insect repellents and pesticides, that can adversely affect the nervous system.
The Pentagon has previously acknowledged that some troops may have been exposed to the nerve agent sarin when Iraqi munitions were destroyed.
The MoD said it was aware of most of the material in the report.
A spokesman said: "We are confident that our approach - which is supported and guided by the Independent Medical Research Council researching Gulf veterans' illnesses - is well focused and deals appropriately with this sensitive issue.
"The findings and the recommendations are of interest, but we note, however, that the report does not take into account the recent paper by the US Institute of Medicine, which states that there is inadequate and insufficient evidence to determine whether an association exists between low level exposure to sarin and long term adverse neurological effects."
Elizabeth Sigmund, of the Gulf Syndrome Study Group, praised the work done in the US.
She said: "The MoD has ignored all these things. They haven't done the sort of clinical research that has been carried out by scientists in the States.
Not stress-related "What the Americans are saying is that the illness is not stress-related and not in the mind.
"It is actual physical damage caused by the chemicals that troops were exposed to."
Labour peer Lord Morris of Manchester, who helped establish the independent British inquiry into Gulf War illnesses under Lord Lloyd, also welcomed the findings.
He said: "This is a major development in unravelling the truth about the lessons of the still medically unexplained Gulf War illnesses.
"The advisory committee is to be congratulated in its frank exposure of the dangers to which the troops were exposed."
Lord Lloyd's inquiry is due to report back within weeks.