Concern about the use of depleted uranium shells by US forces intensified yesterday when Italy asked Nato to investigate claims that six of its soldiers who died after serving in the Balkans were killed by exposure to the munitions.

The request came after an official investigations by France, Spain, Portugal, Belgium, the Netherlands and Finland into the effect of DU weapons.

The Italian prime minister, Giuliano Amato, told the newspaper La Repubblica that alarm about the "Balkan syndrome" was "more than legitimate".

"We've always known that [depleted uranium] was used in Kosovo, but not in Bosnia. We've always known that it was a danger only in absolutely exceptional circumstances like, for example, picking up a fragment with a hand on which there was an open wound, while in normal circumstances it isn't dangerous at all.

But now we're starting to have a justified fear that things aren't that simple."

His defence minister, Sergio Mattarella, said Nato had told Rome only last month that DU had also been used in Bosnia.

US A10 aircraft fired more than 31,000 rounds of DU ammunition in Kosovo. More than 14,000 rounds fell in the area of Kosovo now controlled by Italian troops, according to Italian the deputy ecology minister, Valerio Calzolaio.

DU is a by-product of converting natural uranium into the enriched form used in nuclear weapons and reactors. It is about 40% less radioactive than natural uranium.

The US fired more than 850,000 rounds during the 1991 Gulf war. This has been linked to birth defects in Iraq.

The six Italians who have died since returning from the Balkans all had leukaemia. The latest was Salvatore Carbonaro, 24, from Sicily, who died in November after serving twice in Bosnia but never in Kosovo.

Doctors have said there is insufficient evidence to link the deaths to exposure to DU shells but the Italian media say the number of deaths is too high to be coincidental.

A group representing their families has released a copy of a document in English which it said was a list of Nato guidelines for dealing with DU. It said the document, dated November 22 1999, was not distributed to troops before that date, although soldiers had by then spent months peacekeeping in Kosovo.

Last month the British armed forces minister, John Spellar, admitted that advice on the potential dangerof DU shells failed to reach British troops in the Gulf war.

The Ministry of Defence said yesterday it was not planning to review the effect of DU weapons in the Balkans. It said the radioactivity from the shells was no higher than from household appliances.

The US agency for toxic substances and disease registry had said that no human cancer of any type has ever been seen as a result of exposure to natural or depleted uranium".

Nato sources said yesterday that the North Atlantic Council would discuss the issue at its regular meeting next week.