By Stephanie Nebehay

GENEVA (Reuters) - North Korea said on Tuesday it would pursue its nuclear deterrent and weapons program as huge U.S.-South Korean joint military exercises it says model a "pre-emptive nuclear attack" against Pyongyang continued.

South Korea and the United States, which led condemnation of North Korea's latest missile tests at the Conference on Disarmament, said their military drills were to test defensive readiness against possible aggression from the North.

North Korean diplomat Ju Yong Choi told the United Nations-backed forum that the allies' annual exercises were "a major cause of escalation of tension that might turn into actual war".

"The DPRK (Democratic People's Republic of Korea) is firm in its determination to further bolster its defense capabilities with a nuclear deterrent as a pivot so as to put an end to danger of nuclear war caused by the United States," Ju said.

During the 90-minute session, envoys from more than 20 countries, including North Korea's main ally China, as well as Britain, France, Russia and the United States, condemned North Korea's test-firing of four ballistic missiles on Monday.

Japan's ambassador Nobushige Takamizawa said it was "totally unacceptable" that the missiles had landed off Japan's northwest coast. They posed "a direct and grave threat to Japan's security and a serious danger for aviation and navigation".

U.S. Disarmament Ambassador Robert Wood condemned the tests as North Korea's latest violation of U.N. Security Council resolutions. He described Pyongyang's weapons program as a "clear threat to the national security of every country of the region".

The United States had an "iron-clad commitment to defend its allies" and the military exercises carried out with Seoul since Wednesday were "transparent and defense-oriented", he said.

"It should be very clear to the DPRK that it is a pariah, it is an outlier, it is in violation of multiple U.N. Security Council resolutions, and that the countries represented in this room are not going to stand by and just let the DPRK violate international law," Wood added.

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The United States started to deploy the first elements of its advanced anti-missile defense system known as THAAD (Terminal High Altitude Area Defense) in South Korea on Tuesday, the U.S. Pacific Command said.

"THAAD is purely defensive, that is the only defense measure we can take facing the massive threat posed by DPRK," South Korea disarmament ambassador Kim Inchul said.

But China's ambassador Fu Cong said that Beijing firmly opposed the deployment of THAAD "and will take necessary measures to safeguard its own security interests".

"All parties concerned should work together for the denuclearization of the (Korean) peninsula and its peace and stability," Fu said.

(Reporting by Stephanie Nebehay; Editing by Tom Miles and Catherine Evans)