US Defence Secretary Ashton Carter and his South Korean counterpart vowed zero tolerance Monday for any North Korean provocations and agreed to strengthen combined defences against the myriad threats posed by Pyongyang.

Carter met Han Min-Koo in Seoul during an annual security meeting for the two allies to assess their military cooperation.

Carter told reporters they "spoke candidly" about nuclear weapons, ballistic missiles, cyber- and conventional military threats from North Korea, which they described as a risk to peace and security well beyond the Korean peninsula.

"The minister and the secretary reaffirmed that any North Korean aggression or military provocation is not to be tolerated," according to a joint statement after their talks.

They specifically voiced "grave concern" over strong hints from North Korea that it is preparing a long-range rocket launch in violation of US resolutions.

In a widely expected move, the defence chiefs signed an agreement that defines conditions for a transfer of control over the South Korean military from the United States to Seoul in time of war.

South Korea had been scheduled to take wartime control, known as OPCON, by next year. But now the transfer is based on various conditions being met and not on a particular timeline.

North Korea's hostile rhetoric, rocket tests and unpredictable behaviour in recent years has prompted calls to postpone the transfer. The two nations agreed in principle last year to this "conditions-based" approach.

- Wartime command -

Carter said the main conditions Seoul must meet are the further development of its intelligence capabilities and its counter-artillery powers.

"If we look at global trends in terms of national security, many countries in the world conduct self-defence in the form of cooperation with regional and local partners," Han said when asked why South Korea -- despite its major economic and political clout -- still was not ready to take control of its own military.

The defence chiefs also agreed to strengthen their capacity to deal with cyber attacks.

South Korea, one of the world's most wired nations, has blamed North Korean hackers for a series of cyber-attacks on military institutions, banks, government agencies, TV broadcasters and media websites in recent years.

"We see the North Korean military challenge as continuing to evolve and becoming more complex and more challenging," a senior US defence official said after the meeting, speaking on condition of anonymity.

"Kim Jung-Un's regime has demonstrated a willingness to act provocatively, to use violence, to use force in ways that threaten the stability of the region."

Echoing comments Carter made about the strength of the US-South Korea alliance, the official said ties were at a high point.

"Ten years ago, there were serious disagreements between the two sides on some fundamental issues about the nature of the regime in North Korea," he said.

"The major issues in the alliance now ... are fairly narrow issues about technology transfer and hypothetical contingency situations. We've come a long way."

Carter's visit to South Korea was his first international stop on an eight-day trip to the Asia-Pacific.

He will meet leaders from more than a dozen nations across East and South Asia. Officially, his mission is intended to help push the next phase of America's foreign policy "rebalance" to the region.

A key theme of the trip is likely to be China's construction of artificial islands in the South China Sea and its claims of sovereignty over almost the entire waterway.

The defence chiefs were asked about the issue during a news conference, with Han stressing that any conflict should be resolved "in the framework of international law".

Carter said Sunday the issue was leading many countries in the region "to want to intensify their security cooperation with the United States".

After the meeting in Seoul, Carter left for an Association of Southeast Asian Nations defence ministers' meeting in Kuala Lumpur.