A former prime minister believed to be kidnapped, possibly by a terrorist group, was threatened before he disappeared, Prime Minister Wilfried Martens said Monday.

A caller claiming to represent a previously unknown group, the Revolutionary Socialist Brigade, has said the organization abducted former Prime Minister Paul Vanden Boeynants.Officials said they were handling the case of the 69-year-old Christian Democrat, who disappeared Saturday, as a kidnapping.

"Mr. Vanden Boeynants had received threats lately and had requested to meet with the interior minister," presumably to discuss his safety, Martens said on Belgium's Dutch-langage radio network, BRT. He did not elaborate.

Interior Minister Louis Tobback confirmed he had been due to meet with Vanden Boeynants Monday. He said Vanden Boeynants' secretary called his office Thursday to say her boss wanted to discuss "a personal matter."

However, Deputy Brussels Prosecutor Andre Vandoren told reporters, "We have no elements to say a direct link can be established between the threats to Mr. Vanden Boeynants" and his disappearance.

He said he was treating the disappearance as a kidnapping, possibly for political reasons.

However, Johan Vande Lanotte, an aide to Tobback, said in a statement: "So far, the case is a strictly judicial matter. There is nothing to indicate that a truly terrorist act or politically inspired action is involved."

Three anonymous phone calls have been received claiming responsibility for Vanden Boeynants' disappearance. One was made Saturday to the daily newspaper Le Soir and the others were received Sunday by RTBF, Belgium's French-language radio network.

The calls Sunday to RTBF claimed responsibility for the former prime minister's kidnapping in the name of the Revolutionary Socialist Brigade, an unknown group.

Vanden Boeynants arrived home by car at about 6 p.m. Saturday but never entered his residence, officials said. They said police found his hearing aid, pipe and one of his shoes in his garage.