One of the U.S. sources, who did not want to be identified, said the United States had evidence that Japanese reports about the monitoring of signals suggesting North Korea was preparing a new missile test were accurate.
Both sources said U.S. government experts believed a new test could occur "within days."
A Japanese government source said earlier on Tuesday that Japan had detected radio signals suggesting North Korea may be preparing another ballistic missile launch, although such signals were not unusual and satellite images did not show fresh activity.
Other U.S. intelligence officials have noted North Korea haspreviously sent deliberately misleading signs of preparations for missile and nuclear tests, in part to mask real preparations, and in part to test U.S. and allied intelligence on its activities.
(Reporting by Mark Hosenball; Writing by David Brunnstrom; Editing by Susan Thomas)
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