Communication intercepts in recent days also seem to show that Pyongyang might be planning to launch a mobile ballistic missile in the coming days or weeks, another U.S. official said.
Wednesday, the United States announced it was sending ballistic missile defenses to Guam, a Western Pacific territory that is home to U.S. naval and air bases. North Korea has cited those bases when listing possible targets for missile attacks.
Pentagon officials, while decrying North Korean rhetoric, said recent announcements of U.S. military deployments in response to belligerent statements by Pyongyang may have contributed to the escalating tensions.
As the bombast reaches a fever pitch, the United States is refining its message. The Pentagon now says it is working to decrease the temperature as it maintains a frank and vigilant stance toward the threats.
Starting Wednesday, North Korea also barred South Korean workers and managers from entering the Kaesong industrial complex, an economic cooperation zone that sits on the North's side of the border but houses operations of scores of South Korean companies.
It also repeated a threat from the weekend to shut down the complex, where more than 50,000 North Koreans work.