Depleted uranium (DU) ammunition used by Nato in the mid-1990s in Bosnia-Herzegovina is still polluting air and water there, the UN reports.
By Alex Kirby
BBC News Online environment correspondent
It says there is no cause for alarm, but urges precautions and regular monitoring.
"Tankbuster" A10 aircraft use DU ammunition
Its study has made significant advances in understanding how DU behaves in the environment.
The UN says DU used in Iraq will probably behave in the same way, and needs watching closely.
Coalition forces fighting in Iraq have already used DU ammunition there.
DU is a heavy metal, 1.7 times as dense as lead. It is ideal for punching through armour, and is used mainly for attacking tanks and other armoured targets.
The UK Ministry of Defence (MoD), the Pentagon and Nato say it poses little risk on the battlefield or subsequently, though they say troops entering vehicles struck by DU munitions should wear protective clothing.
Cause for concern
The MoD website says: "We recognise that there could be a small risk to our service personnel from DU dust if they work unprotected close to a vehicle recently hit by DU ammunition."
But the UN has found contamination in Bosnia-Herzegovina from weapons used there in 1994 and 1995.
Its findings are detailed in a report, Depleted Uranium In Bosnia And Herzegovina, published by the UN Environment Programme's Post-Conflict Assessment Unit (PCAU).
Dr Klaus Toepfer, Unep's executive director, said: "These findings must not be seen as a cause for alarm.
"Nevertheless, we recommend that precautions be taken and in particular that ground and drinking water at and near sites where the presence of DU has been confirmed be monitored regularly."
The "new and significant findings" identified by the study are:
The study recommends collecting DU fragments, covering contaminated points with asphalt or clean soil, proper disposal of DU material, keeping records of contaminated sites, and investigating all health claims.
ground contamination occurs at low levels where solid fragments of DU have penetrated, and is limited to about one or two metres around point of impact
the fragments have corroded rapidly, losing 25% of their mass within seven years. They will corrode completely within 25-35 years of impact
this is the first time DU contamination of groundwater has been found. Unep recommends using alternative water sources, with sampling continuing for several years
air contamination was found at two separate sites, including inside two buildings, showing that winds or human activities can disturb DU dust long after the event.
It also wants the missing coordinates of six confirmed attack sites in Bosnia-Herzegovina.
Test-firing DU ammunition in the UK
It is based on the work of experts who investigated 15 sites targeted with DU weapons. Unep chose the sites from data provided by Nato and local authorities.
The team found contamination and fragments at three sites - the Hadzici tank repair base and ammunition storage area, and the Han Pijesak barracks.
Unep says local people and even mine clearance teams "are not sufficiently aware of the risks and issues", and urges a public awareness campaign.
The World Health Organisation (WHO), part of the team, concluded that, "due to the lack of a proper cancer registry and reporting system, claims of an increase in the rates of adverse health effects stemming from DU cannot be substantiated".
Unep says: "The existing scientific data... indicate that it is highly unlikely that DU could be associated with any of the reported health problems."
But Pekka Haavisto, who chairs Unep's DU projects, told BBC News Online: "There are no very comprehensive cancer data, especially from the war years.
"We've always said that if people did inhale the dust for several hours during an attack, you could have a health risk.
"We're a bit concerned to find that we can now measure DU in groundwater, and finding the dust on artillery in the barracks was uncomfortable.
"If DU is used in Iraq I think the consequences will be similar. It's something that should be followed very closely."
The allied forces fired at least 300 tonnes of DU in the 1991 Gulf war.