Hackers have sent emails from Belgium's Prime Minister Elio Di Rupo's personal account to De Morgen newspaper, the daily said on Friday.

Dating from 2004 to 2008 when Di Rupo was president of Belgium's socialist party and before he became prime minister in 2011, the emails were mostly of a private nature, although some did refer to his political activities, the paper said.

It did not publish the emails.

In an interview alongside the report, Di Rupo said he had not decided if he would file a report to police. A spokeswoman for Belgium's Federal Police said once it had a report from him the cyber crime unit would investigate.

"I will examine this thing step by step ... I want to draw lessons out of this bad experience," Di Rupo said.

U.S. Defense Secretary Chuck Hagel said on Friday that cyber threats posed a "quiet, stealthy, insidious" danger to the United States and other nations, and called for rules to guide behaviour and avoid conflict on global computer networks.