The strength of commodities prices, such as crude oil, this year is explained in a large part by speculative factors such as investors piling into the new asset class and the weakness of the US dollar, the International Monetary Fund said on Thursday.

The warning came as commodities prices fell across the board, with oil prices dropping below the $100 a barrel level, gold prices tumbling 10 per cent from their recent record above $1,000 a troy ounce and sharp falls in base metals and grains.