DE Beers, the world's largest producer of rough diamonds by value, on Friday reported a drop in interim sales and a halving of profit because of lower demand.

De Beers is 45% owned by Anglo American, which has offered the Oppenheimer family $5,1bn for its 40% stake.

The Botswana government has an option to lift its stake in De Beers to 25% from 15% by buying part of the Oppenheimer stake. A decision in that regard was expected next month and the deal would be closed by the end of September, De Beers CEO Philippe Mellier said on Friday.

De Beers produced 13,4-million carats in the six months to end-June and it expected its mines in Botswana, SA, Namibia and Canada to produce between 28-million carats and 30-million carats for the full year, he said.

However, De Beers would trim production to its clients' requirements, he said.

De Beers lost three weeks of production from its Jwaneng mine in Botswana after a side of the open-it mine failed on June 29. "We are going to lose some production but we could compensate from other mines," he said.

"Once again we are acting responsibly to lead this industry in a difficult period," Mr Mellier said, referring to a decision by De Beers to idle a number of mines and cut production by half after the onset of the global financial crisis at the end of 2008.

De Beers posted pretax profit of $502m for the period, compared to $1bn a year ago. Its total sales came in at $3,3bn, down from $3,9bn a year earlier.

Its net debt was $980m compared to $1,45bn previously.

There were increased cutting centre stocks, tightening liquidity and difficult conditions in India were affecting short-term demand, Mr Mellier said. "Provided there are no unforeseen economic shocks, De Beers expects to see moderately positive growth in global diamond jewellery sales for the full year 2012 albeit at relatively modest levels."