WASHINGTON - US Defense Secretary Chuck Hagel will announce at 3 p.m. (1900 GMT) on Friday a plan to bolster US missile defenses in Alaska to counter the growing North Korean threat, a US defense official said.The official, who spoke to Reuters on condition of anonymity, did not offer additional details. But a top Pentagon official said on March 12 the United States had the ability to swiftly deploy up to 14 additional ground-based missile interceptors, if needed, in Alaska.