US Defense Secretary Robert Gates said on Thursday that North Korea poses a threat to his country's allies in North Asia and promised to protect them under the US nuclear deterrent.

Gates, on a visit to Seoul, and Secretary of State Hillary Clinton in Washington, said a day earlier the United States would never tolerate a nuclear-armed North Korea, as Pyongyang in recent weeks has indicated it could return to stalled disarmament discussions.

The United States will continue to provide extended deterrence using the full range of military capabilities including the nuclear umbrella to ensure ROK security, Gates said in a meeting with the South's defence chief. South Korea's official name is the Republic of Korea.

Clinton laid down a hard line as Washington weighs whether to hold bilateral talks with North Korea, a step it hopes will bring Pyongyang back to wider, six-party talks on ending its nuclear programmes in exchange for aid and better global standing.

North Korea began reaching out to long-time foes South Korea and the United States after it was hit by fresh UN sanctions for its nuclear test in May aimed at halting its overseas arms sales, which provide vital cash for the destitute state.

South Korean Defence Minister Kim Tae-young said it is too early to say whether reclusive North Korea was ready to engage seriously in nuclear diplomacy.

Although, on the surface, there are signs of some change from North Korea, including its recent willingness to talk, in reality the unstable situation such as the nuclear programme and the military-first policy continues unchanged, Kim said.