Monsanto said it had received no prior warning from the authorities and that all the seeds were conventional. None of the seeds had been distributed to retailers or farmers.  "We assume the authorities will test the seeds for possible GM content,'' a spokesman told Reuters.

The use of genetically modified (GM) seeds in open fields is forbidden by law in Italy, a net importer of maize and soybeans, because of worries over their possible health and environmental impact.

Police have already cracked down on Monsanto in Italy twice before. Between March 23 and 26, they impounded 112 tonnes of maize, and a couple of days later started to seize 300 tonnes of soy seeds that had already been distributed to retailers.

The soy seeds seized on Tuesday have been sealed off at Monsanto's warehouse in Lodi, near Milan, in northern Italy, as were the previous consignments.

Monsanto insisted again on Tuesday that it conformed to regulations.

``We must reaffirm that our seeds are conventional,'' the company said in a statement.

It said the soy seeds had arrived at Genoa port on March 5 and had received all the necessary authorisations.

Last week arsonists attacked Monsanto's grain depot in Lodi, setting maize and soybean seeds ablaze. The assailants, who escaped, painted slogans on walls saying ``No GM'' and ``Monsanto killers.''

After the earlier seizures, Farm Minister Alfonso Pecoraro Scanio asked Milan authorities to suspend Monsanto's seed import licence because he said the company had imported seeds containing GM material.

At the end of last month, Pecoraro Scanio, a member of the Greens party, ordered checks of 21 Italian seed companies, including Monsanto, in a campaign against illegal genetically engineered material.

Tests of Monsanto maize by the Italian Health Ministry last month revealed the presence of the banned genetically engineered Monsanto strain Mon 810.

Last year Italy banned four varieties of GM maize, saying they were not ``substantially equivalent'' to conventional maize.

Italy has zero tolerance towards GM seeds, even though the EU Scientific Committee on Plants and other groups say the presence of GM material in seeds is inevitable because of unintentional contamination in the production process.

An official familiar with the seizures of Monsanto grain said EU legislation was unclear over how to tackle cases where grain labelled as conventional was found to contain genetically engineered material.

It was up to individual governments to decide how to proceed in such cases, he said.