By Phil Stewart

WASHINGTON (Reuters) - Defense Secretary Chuck Hagel will announce at 3 p.m. (1900 GMT) on Friday a plan to bolster U.S. missile defenses in Alaska to counter the growing North Korean threat, a U.S. 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 Tuesday the United States had the ability to swiftly deploy up to 14 additional ground-based missile interceptors, if needed, in Alaska.

Hagel's expected announcement will come a week after North Korea threatened the United States with a pre-emptive nuclear strike and a month after Pyongyang conducted its third nuclear test, in defiance of U.N. resolutions. It conducted a launch of a long-range rocket in December.

Experts say North Korea is years away from being able to hit the continental United States with a nuclear weapon despite a decades-long push toward a nuclear capability.

But its fiery rhetoric and aggressive testing have increased tension with the United States and South Korea.

"North Korea's shrill public pronouncements underscore the need for the U.S. to continue to take prudent steps to defeat any future North Korean ICBM," James Miller, the Defense Department's policy chief, told the Atlantic Council on Tuesday.

Miller - also citing tensions with Iran - said the Pentagon was initiating congressionally mandated environmental impact studies for three alternative sites for deploying additional ground-based interceptors, if needed.

"These studies will allow us to shorten the timeline to build a new missile field on the East Coast or to add interceptors in Alaska, should either approach become necessary due to further future increases in the threat from Iran and North Korea," Miller said in his address.

(Reporting by Phil Stewart; Editing by Eric Beech)