France's agriculture minister warned on Friday that "hundreds of thousands" of ducks would still have to be culled to halt a severe strain of bird flu devastating flocks in southwest France.

More than 400,000 ducks have already been slaughtered mainly in the Landes department, a bastion of France's foie gras industry, and at least 200,000 more have already been announced.

"There will be hundreds of thousands more, that's certain," Julien Denormandie told France Bleu Gascogne radio ahead of a visit to farms hit by the outbreak, calling it "a race against the clock" against the "highly pathogenic" H5N8 virus.

On Thursday, officials extended the cull of all ducks and geese to within a five-kilometre radius of infected flocks from three, while also requiring additional buffer zones where no poultry could enter.

"It's a drastic move," Denormandie said, adding: "It's clear that we didn't act fast enough."

He acknowledged that he did not know how many ducks would remain among the five million currently being produced in the Landes, but promised that compensation based on their market value would begin to arrive next week.

Foie gras producers fear a repeat of the massive bird flu outbreak that struck in the winter of 2015-16, when more than 25 million ducks were culled. That was followed by an outbreak the next winter that saw 4.5 million ducks killed.

Belgian authorities said Thursday the virus had also been found in three flocks that were culled, and cases have also been reported in Britain, Ireland, the Netherlands and Sweden.

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GASCOGNE