"These [Germany and the United States] are friendly countries, allies. The question of trust is an important issue and if it so happens that the facts are proven, the government will take very tough initiatives on the matter," Michel said.
Last month, Belgian Deputy Prime Minister Alexander de Croo said that Germany would "have some explaining to do" if the reports about spying in Belgium are proven correct.
In April, German media uncovered internal intelligence investigation documents, which appeared to indicate that BND had been spying on high-ranking European officials and defense companies on behalf of the the NSA.
Last year, Germany opened an inquiry into claims that the NSA eavesdropped on German Chancellor Angela Merkel’s mobile phone, a revelation that brought a chill to German-US relations.