The government has asked Washington to confirm whether Korean presidents were among 35 world leaders who were the target of wiretapping by the U.S. National Security Agency in 2006.

A Foreign Ministry official said the Korean Embassy in Washington has sought clarification from the U.S. government.

The allegation arises from an investigation by a German magazine that revealed that German Chancellor Angela Merkel had been the victim of wiretapping by the NSA for a decade and that 34 other world leaders were also targeted.

If the allegations are confirmed, the official warned Seoul will take "strong" measures.

After a flood of revelations that U.S. secret services snooped into the electronic communications of billions of people around the world, including close allies, the U.S. government now faces demands for a radical overhaul of its information-gathering activities.

But the official said Seoul is merely seeking clarification and has no evidence that Korean presidents were bugged. The official added Washington has not yet responded and it is unclear whether it will.

Glenn Greenwald, the Guardian reporter who is working on documents leaked by NSA whistleblower Edward Snowden, has said Korea was on the list of countries whose leaders were targets of electronic eavesdropping.

