The United States says it is not interested in negotiations with North Korea "for the sake of simply talking," stressing the country must stop all nuclear activity.
The U.S. statement to the U.N. nuclear agency on Wednesday said North Korea must "demonstrate its seriousness on denuclearization."
The statement follows an International Atomic Energy Agency report that strengthened suspicions North Korea turned to black market suppliers to set up a uranium enrichment plant.
Last week a U.S. scientist who visited a previously secret North Korean uranium enrichment plant said it was more advanced than operations in Iran and that Pyongyang might launch a third atomic test.
The North tested nuclear devices in 2006 and 2009.