By Stephanie Nebehay

GENEVA (Reuters) - North Korean Foreign Minister Ri Su Yong said on Tuesday that his country had the power to deter an "ever-increasing nuclear threat" by the United States with a pre-emptive strike if necessary.

His rare speech at the U.N.-backed Conference on Disarmament drew a rebuke from U.S. Ambassador Robert Wood, who urged Pyongyang to stop making threats and rid itself of nuclear weapons.

Ri said joint military exercises currently being staged by South Korea and the United States were "unprecedentedly provocative in nature and have an especially high possibility of sparking off a war."

"The DPRK (Democratic People's Republic of Korea) cannot but bolster its nuclear deterrent capability to cope with the ever-increasing nuclear threat of the U.S.," he told the Geneva forum. "Now the DPRK has the power of deterring the U.S. and conducting a pre-emptive strike as well, if necessary."

North Korea fired two short-range missiles off its eastern coast on Monday, South Korean officials said, in a move seen as a defiant response to this year's U.S.-South Korean military exercises. Pyongyang regularly denounces the annual drills, which it views as a preparation for war.

The missiles landed in the sea between the Korean Peninsula and southern Japan early on Monday after travelling for about 490 km (305 miles), according to South Korea's Defence Ministry.

Ri, speaking in Korean, did not refer to the firing.

Takashi Uto, Japan's parliamentary vice-minister for foreign affairs, told the forum the missile firing was a "clear violation" of U.N. Security Council resolutions.

U.S. envoy Wood said the exercises with South Korea, held for almost 40 years, were "transparent and defence-oriented".

"We call on the DPRK to immediately cease all threats, reduce tensions and take the necessary steps towards denuclearization needed to resume credible negotiations," Wood said, referring to six-party talks that collapsed in 2008.

Story continues

"We will not accept North Korea as a nuclear-armed state and we will do what is necessary to defend ourselves and our allies," he added.

Later at the U.N. Human Rights Council, Ri rejected the findings of an inquiry last year based on testimony from defectors and people still in North Korea. Its report said violations committed by the state may amount to crimes against humanity and called for prosecutions.

"The hostile forces are only interested to hear from such scum of mankind as the so-called 'defectors from the North' who fled abandoning their parents, wives and children after committing crimes at home," Ri said.

He called on the forum to address "brutal torture crimes" he said were committed by the U.S. Central Intelligence Agency.

(additional reporting by Tom Miles; Editing by Tom Heneghan)