Levees are being fortified in the Midwest where the swollen Mississippi River will peak over the weekend.

The area has been hit by the worst flooding in 15 years, with thousands of acres of prime farmland in the heart of the world's largest grain exporter already inundated.

The Mississippi has breached or overtopped some two-dozen levees so far this week, with 25 more seen at risk before its expected crest near St. Louis, Missouri, on Sunday.

The cost of the flooding across the US corn belt will be felt by consumers worldwide in terms of higher food prices, and in business losses yet to be totalled.

Flooding and violent storms have been blamed for 24 deaths since late May, and some 40,000 people have been forced from their homes.

Ultimately, the cost of the disaster may rival that of those 15 years ago that caused more than $20 billion (£508 million) in damage and 48 deaths.

President George W Bush visited flooded Iowa cities as his administration promised relief would be made available from the $4 billion (around £2 billion) in the government's disaster fund.

"I know a lot of farmers and cattlemen are hurting right now," Mr Bush said at an emergency centre in Cedar Rapids, Iowa, among the cities hit hardest.

Volunteers, National Guard troops and prison inmates have joined in the effort to shore up strained and leaking levees protecting homes and thousands of acres of prime farmland.

The flooding in five Midwest states was thought to have ruined at least 5 million acres.

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