North Korea Monday renewed its threat to wage nuclear war and vowed to respond to an 11-day South Korean-U.S. military exercise called Key Resolve/Foal Eagle.

   Speaking to reporters, State Department spokesman Philip Crowley said the exercises are "defensive in nature."

   "The U.S. and Republic of Korea routinely conduct joint military exercises," Crowley said. "North Korea was notified about these exercises on February 14th, and its belligerent rhetoric is unwarranted."

   The U.S. pulled all of its tactical nuclear weapons out of South Korea in 1991 as the two Koreas signed an agreement calling for denuclearization of the Korean Peninsula and inter-Korean rapprochement.

   Washington since then has committed to providing so-called "extended deterrence," using all of the U.S. military might, including the nuclear umbrella and ballistic missiles, in defense of South Korea.

   Some South Korean conservatives, however, have periodically called for the redeployment of U.S. tactical nuclear weapons or a nuclear-armed South Korea since North Korea conducted nuclear tests in 2006 and 2009 and test-fired ballistic missiles.

   Then-South Korean Defense Minister Kim Tae-young said in November that he would consider discussing with the U.S.

redeploying U.S. tactical nuclear weapons back to South Korea, although his remarks were quickly withdrawn by the presidential office, Cheong Wa Dae, and the Defense Ministry.

   Nuclear-armed North Korea has made a series of provocations in recent months, including the sinking of a South Korean warship and shelling of a border island that killed 50 people last year.

   In November, the regime disclosed a uranium enrichment plant that could be used to make nuclear weapons apart from its plutonium program. The North claims its intention is to generate electricity.

   South Korea and the U.S. want the U.N. Security Council to address the North's uranium program before any resumption of the six-party talks on ending Pyongyang's nuclear weapons ambitions.

   China, North Korea's staunchest communist ally, opposes any idea of the Security Council dealing with the uranium program, citing a lack of concrete evidence and its possible adverse impact on an early resumption of the multilateral nuclear talks, involving the two Koreas, the U.S., China, Japan and Russia.

   The talks have been deadlocked for more than two years over the North's missile and nuclear tests and attacks on the South Korean warship Cheonan and Yeonpyeong Island.

   Seoul and Washington also demand Pyongyang apologize for the provocations before moving to the denuclearization-for-aid talks, while Beijing wants the multilateral nuclear talks to reopen as soon as possible without any conditions attached.

   Inter-Korean military talks broke down earlier this month as the North balked at Seoul's demand for an apology for last year's provocations, dampening the reconciliatory mood Pyongyang has employed recently to attract food and economic aid through the six-party talks.

   U.S. Defense Secretary Robert Gates said last month that North Korea's missiles and nuclear weapons will pose a threat to the U.S. within five years.

   The North is believed to have at least several nuclear weapons, with some experts saying it may have already developed nuclear warheads small enough to be mounted on ballistic missiles.

   Reports said that North Korea is digging a new tunnel to prepare for a third nuclear test and that it has completed the construction of a new, sophisticated missile launch site on its western coast near the Chinese border in an apparent bid to test-fire another ballistic missile that can reach the mainland U.S.

   The U.S. fears any redeployment of tactical nuclear warheads in South Korea or the South's nuclear armament will trigger a nuclear arms race in Northeast Asia and undermine international efforts to denuclearize the North through the six-party talks.

   The U.S. maintains a nuclear cooperation agreement with South Korea that bans Seoul from enriching uranium or reprocessing spent nuclear fuel.

   Late South Korean President Park Chung-hee sought a clandestine project for the development of nuclear weapons in the 1970s to cope with military threats from the North after the U.S. took steps to reduce its troops in Korea.

   Park's ambitions were thwarted by the U.S., which successfully pressured France and Canada to refrain from helping South Korea build nuclear reactors capable of producing weapons-grade material.

   The aborted South Korean program was described in a report by the James Martin Center for Nonproliferation Studies in March last year.

   "This initial effort was halted, however, after the 1974 Indian 'peaceful' nuclear test prompted the United States to turn against the spread of reprocessing technologies and after revelations that the then-military government of Korea was planning to develop nuclear weapons or, at least, acquire the technology and capability to do so on short notice," the report said.

   South Korea and Japan are said to have enough technology to develop nuclear weapons.

   "Several friends or allies of the United States, such as Japan and South Korea, are highly advanced technological states and could quickly build nuclear devices if they chose to do so," a report of the U.S. Joint Forces Command said in February last year.

(Yonhap News)