Depleted uranium (DU) ammunition used by Nato in the mid-1990s in Bosnia-Herzegovina is still polluting air and water there, the UN reports.
It says there is no cause for alarm, but urges precautions and regular monitoring.
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."
"
If DU is used in Iraq I think the consequences will be similar
"
Pekka Haavisto, Unep
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:
It also wants the missing coordinates of six confirmed attack sites in Bosnia-Herzegovina.
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.
Risks unproven
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.
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