General Motors says its US car sales have fallen in June, and early indications are that they will be below previous estimates.

The fall has been attributed to the combination of slower sales and high stocks of cars and light trucks, which meant dealers had to push up the value of special deals for consumers.

As a result of dwindling sales in the world's largest car market, GM started offering $5,000 cashback on most of its sports utility vehicles to its existing GM customers.

At an Automotive News conference in Switzerland, GM's Chairman of North American operations, Bob Lutz said: "It seems like, compared to May, demand has slackened and in that environment incentives tend to rise."

Stocks had been allowed to rise earlier in the year as predictions of higher sales were made, because of increased consumer spending. However, as Mr Lutz said: "We were all planning on a surge in demand that never quite materialised."

Some analysts have voiced optimism about the market, arguing that the dip in demand is only a temporary setback.

Changes in vehicle production levels may affect the volumes of platinum, palladium and rhodium bought by the auto industry for use in autocatalysts to control harmful emissions.