"The investigation in the house of Mr Claes is the beginning of a new round that shouldn't last too long," said Hans van Mierlo after a meeting of EU foreign ministers in Luxembourg.
Mr Claes's official and private residences were searched by police last week. "This strongly underlines his vulnerability," Mr van Mierlo said.
Mr van Mierlo had previously indicated that he had doubts about Mr Claes's position, but this statement is the strongest yet from a Nato government over the Secretary-General. Mr Claes has admitted that he was told of an offer of cash from Agusta, the Italian helicopter company, in return for a sale to the Belgian army.
At the weekend, Karl Lamers, the foreign policy spokesman for Germany's ruling Christian Democrat party, said that he believed the search undermined Mr Claes's position. Mr Lamers is very close to Chancellor Helmut Kohl.
British sources were yesterday little more than lukewarm in their support of Mr Claes. However, Britain would be constrained from publicly disputing Mr Claes's position because it has previously supported him.