Europe's busiest seaport, Rotterdam, has expanded its anchorage area to accommodate rising demand from oil traders storing fuel at sea and more idling ships awaiting cargoes.

Dutch waterways authority Rijkswaterstaat designated an area of 15 kilometres northwest off the North Sea resort of Scheveningen as a new anchoring zone, the port authority said in a statement on its website Friday.

The new area lies along the shipping route to Scandinavia and the Baltic countries and can anchor about 15 vessels, it said.

''Many tankers are lying at anchor here to wait for orders or for speculative considerations. The popularity of North Sea anchoring spots has increased considerably,'' the authority said.

The combined capacity of ships storing oil products at sea has risen more than fivefold since April, according to Simpson Spence & Young Ltd, the world's second largest shipbroker. About two-thirds of those cargoes are in Europe.

Some merchant ships are also anchoring because they have no cargoes to make any voyages.

Oil traders can purchase the cargoes, store them and sell them at higher prices through derivatives contracts to lock in profit.

In Europe, vessels are also anchored off the UK coast at locations including Southwold, to the east.

According to local media reports, the number of idling vessels off Rotterdam has increased to an average of 50 to 60 with peaks of 90, compared to 40 before the economic crisis.

Rotterdam handles some 401.2 million metric tonnes of cargo in 2007, more than twice the second largest European port Antwerp, according to a ranking by the America Association of Port Authorities.