By Park Si-soo

North Korea and the United States held a "secret" meeting in Switzerland this week, Japanese broadcaster NHK reported Thursday. The meeting took place on the sidelines of an international security forum there from Monday to Wednesday.

Choe Kang-il, deputy director general for North American affairs at the North's Foreign Ministry, and Evans Revere, a former principal deputy assistant secretary of the U.S. State Department, held the talks, according to NHK.

It is unknown what they discussed, but it was very likely to have been about the heightened tensions after Pyongyang's sixth nuclear test.

Choe remained silent when he faced a barrage of questions from reporters while leaving the meeting venue, according to NHK.