U.S. Defense Secretary Robert Gates said Thursday that North Korea continues to pose a grave threat to international peace and pledged to maintain a nuclear deterrent in the region.

Gates and his South Korean counterpart, Kim Tae-young, said their two nations would never accept North Korea as a nuclear weapons state. They accused the regime of undermining global security with atomic and missile threats, and said recent overtures from Pyongyang do not diminish the serious atomic threat.

Ballistic missile and nuclear weapons tests conducted in April and May "clearly violate" U.N. Security Council resolutions and international disarmament agreements, they said in a joint statement following their talks.

The violations "undermine the global nonproliferation regime and constitute direct and grave threats to peace and stability" not only for South Korea and the region but also for the broader international community, they said.

Earlier, Gates reaffirmed the U.S. commitment to defending ally South Korea.

"I want to reaffirm the unwavering commitment of the United States to the alliance and to the defense of the Republic of Korea," he said before going into talks with Kim. "The United States will continue to provide extended deterrence, using the full range of military capabilities, including the nuclear umbrella" to ensure South Korea's security.

North Korea sees Washington's nuclear deterrent as a key threat to its survival and long has maintained that it needs its own atomic program to defend itself against the U.S. nuclear umbrella.

Gates and Kim said they agreed to cooperate closely on implementing two U.N. Security Council resolutions that seek to stop North Korea from engaging in ballistic missile activity and in working toward bringing North Korea back to disarmament talks that involve Russia, Japan, China, the U.S. and the two Koreas.

The strong stance from the top defense officials comes amid some signs of possible softening by North Korea after months of tension over Pyongyang's nuclear and missile programs.

North Korean leader Kim Jong Il said earlier this month that his country could rejoin six-party nuclear talks, depending on the status of direct talks with the U.S.

North Korea's No. 2 nuclear negotiator, Ri Gun, was on his way to the U.S., where he is scheduled to attend a security forum next week in California and a seminar in New York. Ri also reportedly is expected to meet with the chief U.S. nuclear negotiator, Sung Kim, to set up bilateral talks.