Trading volume surged for exchange-traded foreign currency futures in August, a period of heightened market volatility due to fears about U.S. and European sovereign debt and a weakened global economy.

CME Group Inc. (CME) reported Friday that an average 991,000 foreign exchange contracts changed hands during August, up 11% from July and 21% higher from the same period a year earlier.

The average notional value of forex contracts traded each day at CME was $137 billion, up from $125 billion in July.

Over-the-counter transactions away from exchanges dominate currency trading, although CME sees forex as a significant growth opportunity.

Exchange-wide, August was the busiest month ever at CME, with average daily volume of 17.1 million contracts.

Forex on its own did not post record volume in August, according to CME. The all-time high was in May 2010, when the stock market plunged in the so-called flash crash. Average daily forex volume was about 1.3 million contracts that month, with a notional value of $160 billion, a CME spokesman said.

CME's Atlanta competitor, IntercontinentalExchange Inc. (ICE), lists futures on the closely watched U.S. dollar index, which measures the dollar's performance against a basket of other currencies.

ICE on Friday said average daily volume on the dollar index was at almost 28,000 or about 641,000 for all of August, up 10% from July.

The exchanges, which collect fees for each traded contract, benefited from volatility caused by Standard & Poor's historic ratings downgrade of U.S. government debt. In Europe, some countries faced the prospect of defaulting on their debt, endangering the sustainability of the unified euro currency.

Fiscal austerity measures threaten to slow economic growth in Europe, while several months of weak economic data have the U.S. teetering on a double-dip recession.

At CME, trading volumes climbed for futures on the Japanese yen and the Swiss franc, currencies currently regarded as safe havens, and each targeted for possible central bank intervention.

CME reported a 33.5% volume jump for futures on the yen, compared to July. Futures on the Swiss franc rose 17.3% during August.

The euro is the most actively traded currency at CME, with almost 8.1 million contracts traded last month. That is up 16.7% from July.