Jean-Claude Trichet, president of the European Central Bank, on Wednesday called for concerted market action to improve transparency in global credit derivatives markets, where he said “opacity” made it difficult to assess risks to the financial system.

The private, over-the­-counter nature of derivatives markets makes them notoriously difficult to measure and monitor, but the explosive growth in the use of credit derivatives continues as increasing numbers of traditional asset managers and pension funds join investment banks and hedge funds in the market.