"And, as such, I want to reaffirm the unwavering commitment of the United States to the alliance and to the defence of the Republic of Korea [South Korea]."

The US stations 28,500 troops to assist South Korea's 655,000-strong armed forces against the North's 1.2 million-strong army. It also guarantees a "nuclear umbrella" over its ally in case of a nuclear attack.

Kim said the North's policy had not changed.

Although "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 a military-first policy continues unchanged," he said.

In a joint statement, Gates and Kim said the North's missile and nuclear tests in April and May, along with recent short-range missile tests, violate UN Security Council resolutions.

'Grave threat'

North Korea test-fired short-range missiles despite recent diplomatic overtures [AFP]
"They also undermine global non-proliferation efforts and constitute direct and grave threats to regional and international peace," it said.

At a news conference on Thursday, Gates also reiterated that Washington would stand together with other allies and partners in seeking the North's "complete and verifiable denuclearisation".

After months of tension, the North began making peace overtures to Seoul and Washington in August. However, it still test-fired short-range missiles last week and warned South Korea of a potential naval clash on their disputed border.

The North quit six-party nuclear disarmament talks in April. The government also indicated a willingness to return to multilateral talks but only if it first holds discussions with the United States alone.