German authorities arrested a 54-year-old Swiss citizen on suspicion of spying Friday, but declined to say who he allegedly worked for.

Federal prosecutors said the man, identified only as Daniel M. in line with German privacy rules, was detained Friday in Frankfurt, the country's financial capital, on the basis of an arrest warrant issued Dec. 1.

In a statement, prosecutors said M. is "suspected of working for the intelligence agency of a foreign power since the start of 2012."

Markus Schmitt, a spokesman for the prosecutors' office, declined to say which foreign power was allegedly involved.

Prosecutors said M. would appear before a judge Friday.

Switzerland's foreign ministry said it had "taken note" of the arrest of a Swiss citizen but declined to comment further, citing privacy rules.