LISBON, Portugal -- Portugal has told NATO it must back up its assurances that traces of plutonium, found in shells and bullets used in Yugoslavia, pose no risk to health.

Prime Minister Antonio Guterres believes the credibility of the military alliance is at stake, with public opinion already inflamed over the issue of depleted uranium ammunition.

And he has written to Secretary-General George Robertson warning NATO must demonstrate that the plutonium, even in the "negligible quantities" admitted by the alliance, posed no special danger.

"For the credibility of the alliance, it is of the utmost importance ... that the explanations I am asking fully demonstrate that the use of such ammunitions could not cause any health hazards both in NATO troops and the local population," he wrote.

NATO confirmed on Thursday that faint traces of radioactive uranium 236 and plutonium could be present in some depleted uranium (DU) ammunition fired during the Balkans conflicts of the 1990s.

But spokesman Mark Laity said independent experts agreed the levels found were so low as to be meaningless in health terms.

The admission that plutonium had been detected has added to the furore surrounding the use of DU ammunition.

Some ailing soldiers and anti-nuclear campaigners say the armour-piercing rounds, used in Bosnia and Kosovo, have caused cancer and death.

NATO insists there is no scientific evidence to back up those claims and no evidence of any group of symptoms among former peacekeepers that could describe a common "Balkans Syndrome".

But Portugal and other NATO members have called for a full investigation into the health issues surrounding DU ammunition and some alliance states want a moratorium on its use until all the implications are clear.

Portugal, where a soldier died of brain disease after service in Kosovo, sent a team of experts to Kosovo and Bosnia to test for radiation in areas where its peacekeeping troops are stationed.

And Germany has begun its own investigation. Defence Minister Rudolph Scharping has demanded that the United States come forward with all the information it has on the composition of the DU weapons.

Italy has urged the United Nations to expand its investigation on the issue after raising from seven to eight the number of people it believes may have died after possible exposure to depleted uranium ammunition.

Italian Defence Minister Sergio Mattarella told parliament that illnesses suffered by a further 23 soldiers are also being looked at.

Depleted uranium, a slightly radioactive heavy metal, is used in anti-armour munitions because of its high penetrating power.

U.S. forces fired weapons containing depleted uranium in Bosnia in 1994 and 1995, and in 1999, NATO fired such weapons during its 78-day bombing campaign in Yugoslavia.

The Associated Press & Reuters contributed to this report.