A Nato spokesman said bombs had been dumped on several occasions.

"The procedures were carried out in international waters in 'designated areas' for this sort of operation," he added.

BBC News' Rachael Payne reports

The bombs are thought to have been dropped by some of the hundreds of Nato planes flying missions over the Balkans.

The major Aviano air base is close to Venice and it is standard practice to jettison weapons if a military plane is in difficulty as it comes in to land.

Italian army officials who examined the devices said they probably dated from World War II.

But Nato admitted some bombs could have been prematurely dropped by pilots because of mechanical problems.

Strike action

David Willey reports from Rome: "Growing public disquiet"

During the past week two fishermen were injured in explosions and about 30 bombs were caught in fishing nets.

Venice fishermen have been on strike since Wednesday to try to force the Italian authorities to dispose of the bombs.

In a meeting between Italian authorities and about 200 fishermen in Chioggia on Saturday, national fishing co-operative president Enzo Fornaro said he had never heard of designated areas.

"It would be absurd because once explosives are dropped in the water they risk drifting to other parts of the sea," he said.

Nato military spokesman Major General Walter Jertz stressed that the bombs were found in international waters rather than Italian territory.

The incident has stirred further anti-Nato sentiment in Italy, which has seen several demonstrations against the Kosovo campaign.

Italian opposition leaders are planning a peace march to Assissi - the birthplace of St Francis - on Sunday.