By Phil Stewart and David Alexander

WASHINGTON, March 15 (Reuters) - Defense Secretary Chuck Hagel announced plans on Friday to bolster U.S. missile defenses in response to a growing nuclear threat from North Korea, adding 14 interceptors to a missile defense site in Alaska by 2017 and deploying a radar tracking system in Japan.

The decision to add 14 new anti-missile interceptors at Fort Greely in Alaska amounts to a reversal of an Obama administration decision in 2010 to stop expansion of the missile field there at 30 interceptors. The Bush administration had planned to deploy 44 total interceptors.

Hagel said the decision to deploy all 44 interceptors came as a result of the growing threats from Iran and particularly North Korea, which tested a third nuclear device last month and launched a rocket that put a satellite in orbit in December.

"The reason that we're doing what we're doing ... is to not take any chances, is to stay ahead of the threat," Hagel said.

He said the additional interceptors would be deployed by the end of 2017, but did not say when their deployment would start.

Hagel also said the United States would move forward with a plan announced by his predecessor last year to deploy a second missile defense radar in Japan.

Hagel's announcement came a week after North Korea threatened the United States with a pre-emptive nuclear strike.

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," or intercontinental ballistic missile, 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 Mohammad Zargham)

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  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un greets military personnel at a long-range artillery sub-unit of KPA Unit 641 during his visit to front-line military units near the western sea boarder in North Korea near the South's western border island of Baengnyeong. (AP Photo/KCNA via KNS)

  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un waves at military officers after inspecting the Wolnae Islet Defense Detachment, North Korea, near the western sea border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un, center, confers with military officers at a long-range artillery sub-unit of KPA Unit 641 during his visit to front-line military units near the western sea border in North Korea near the South's western border island of Baengnyeong. (AP Photo/KCNA via KNS)

  • In this undated photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un, second left, watches sturgeons in a pond at the Ryongjong Fish Farm in South Hwanghae, southwestern North Korea. (AP Photo/KCNA via KNS)

  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un rides on a boat, heading for the Wolnae Islet Defense Detachment, North Korea, near the western sea border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 11, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 12, 2013 by the Korea News Service, North Korean leader Kim Jong Un, third left, looks at South's western border island of Baengnyeong during his visit to the Wolnae Islet Defense Detachment, North Korea. (AP Photo/KCNA via KNS)

  • In this March 7, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 8, 2013 by the Korea News Service, North Korean leader Kim Jong Un, center, is welcomed by military personnel at a military unit on Jangjae islet, located in the southernmost part of the southwestern sector of North Korea's border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 7, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 8, 2013 by the Korea News Service, North Korean leader Kim Jong Un, center, walks with military personnel as he arrives for a military unit on Mu Islet, located in the southernmost part of the southwestern sector of North Korea's border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 7, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 8, 2013 by the Korea News Service, North Korean leader Kim Jong Un, center, uses binoculars to look at the South's territory from an observation post at the military unit on Jangjae islet, located in the southernmost part of the southwestern sector of North Korea's border with South Korea. (AP Photo/KCNA via KNS)

  • In this March 7, 2013 photo released by the Korean Central News Agency (KCNA) and distributed March 8, 2013 by the Korea News Service, North Korean leader Kim Jong Un, with military officials, gets a ride on a boat on his way to a military unit on Jangjae Islet, located in the southernmost part of the southwestern sector of North Korea's border with South Korea. (AP Photo/KCNA via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) meeting with family of a soldier as he inspects Jangjae Islet Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) inspecting Jangjae Islet Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) waving to soldiers from a wooden boat as he inspects the Mu Islet Hero Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) meeting with family of a soldier as he inspects Jangjae Islet Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) inspecting Jangjae Islet near from South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) inspecting the Mu Islet Hero Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)

  • This picture, taken by North Korea's official Korean Central News Agency on March 7, 2013 shows North Korean leader Kim Jong Un (C) posing with soldiers and their family members as he inspects Jangjae Islet Defence Detachment near South Korea's Taeyonphyong Island in South Hwanghae province, North Korea's southwestern sector of the front. (KCNA/AFP/Getty Images via KNS)