"We need to ask what are the legal frontiers in this case and whether it is right that a U.S. civil servant could look at private transactions without the approval of a Belgian judge," government spokesman Didier Seus said Monday.
By contrast, the European Union -- which has probed allegations about secret CIA prisons and other activities undertaken in Europe by the United States as part of its self-declared war on terrorism -- is steering clear of the SWIFT controversy.
"At first sight there is no European legislation covering this type of transfer and it is therefore a matter of national law," said Friso Roscam Abbing, a spokesman for Justice and Home Affairs Commission Franco Frattini.
U.S. officials acknowledged last week that the Treasury Department had been accessing the records held by the Society for Worldwide Inter-bank Financial Telecommunications, known by its acronym SWIFT, since September 2001. The records form a database of essentially every international financial transaction, and reports say that U.S. investigators were trawling it with sophisticated software, looking for evidence of financial activity by terrorist groups.