The brief visits in April and August were aimed at encouraging the new leadership to moderate its foreign policy after the death of Kim’s father, longtime autocrat Kim Jong Il, in December 2011.

The ruling elite apparently spurned the outreach effort, however. This month, after a surge of fierce anti-U.S. rhetoric, the government in Pyongyang defied international warnings and conducted its third and most powerful underground nuclear test.

The former U.S. officials, who spoke on condition of anonymity because the back-channel trips have not been formally disclosed, said the first visit was an unsuccessful attempt to persuade Pyongyang not to launch a long-range rocket.

North Korea carried out the launch April 12. The missile flew only a few minutes before it exploded and crashed into the sea. A subsequent test of another long-range rocket in December was successful.