FRANKFURT, July 9, 2008 (AFP) - Opaque sovereign wealth funds in emerging economy nations complicate an evaluation of the euro's role as a global reserve currency, the European Central Bank said Wednesday.

'In terms of publicly available data ... sovereign wealth fund assets held by developing countries are less transparent than traditional foreign exchange reserves,' the ECB said in a report on the euro's international role.

Few of these funds release data, the ECB said, noting that countries including China and Russia have transferred some of their huge foreign currency reserves into the funds.

China alone holds reserves of nearly 1.7 trillion dollars (1.07 trillion euros).

State-owned funds based in oil producing countries in the Middle East, in Norway, Russia and China, manage between two and three trillion dollars at present.

That represents between one-third and one-half of traditional global reserves and the percentage is likely to grow rapidly in the years to come, the ECB said.

'Efforts to increase transparency would help to monitor the development of the role of the euro in this area,' the bank said, estimating that the euro's share in global foreign reserves was 26.5 percent at the end of December, up 1.4 percentage points from a year earlier.

However, when corrected for foreign exchange effects, the portion of foreign reserves held in euros was in fact down 1.5 percentage points, it added.

The bank said the single currency's role grew during the first five years of monetary union before levelling off in the subsequent five years.

The dollar is still by far the leading reserve currency, at around 63.9 percent of the total, while the pound represents 4.7 percent and the yen 2.9 percent, the ECB said.

In other areas, the euro has slipped back slightly on bond markets, possibly owing to 'turmoil in global financial markets since mid-2007,' when the US market for high risk, or subprime mortgages collapsed, ECB president Jean-Claude Trichet said.

On foreign exchange markets use of the single European currency has remained stable although the amount of euros in circulation outside the eurozone has increased slightly, essentially in eastern European countries and in Asia.