SEOUL, October 21, 2009 (AFP) - The United States will never accept a nuclear-armed North Korea, Defense Secretary Robert Gates warned Wednesday, saying its atomic and missile programmes pose a "lethal and destabilising" threat.

"There should be no mistaking that we do not today, nor will we ever, accept a North Korea with nuclear weapons," Gates told US and South Korean soldiers as he began a visit to Seoul.

"The peril posed by the North Korean regime remains, and in many ways has become even more lethal and destabilising," he said in a speech.

Pyongyang's pursuit of nuclear weapons and "proliferation of nuclear know-how and ballistic missile weapons and parts" threaten not just the region but international stability, Gates said.

North Korea quit six-party nuclear disarmament talks in April and staged its second atomic weapons test the following month. It test-fired a barrage of ballistic missiles in July.

The communist state has indicated its willingness to return to multilateral talks, but only if it first holds discussions with the United States alone.

South Korea and other six-party members are wary of the North's attempts to sidetrack the forum, which groups the two Koreas, the United States, China, Russia, and Japan.

Gates said Washington would work with international partners to ensure the North abandons its nuclear weapons programme, and would bolster its regional allies against the threat from Pyongyang.

It was committed to "providing extended deterrence using the full range of American might -- from the nuclear umbrella to conventional strike and missile defense capabilities," he told the troops at the US Yongsan base in an uncompromising message.

"At the outset I want our Korean allies to know that America will continue to stand with you, shoulder to shoulder, as a close friend and reliable partner."

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

A top US military commander, attending security talks with South Korean officers in Seoul, portrayed the North as volatile and unpredictable.

"A nuclear-armed North Korea, and a North Korea that chooses to provoke, and a North Korea that may be on the brink of succession -- all of those things make North Korea certainly worthy of our attention now," said Admiral Robert Willard, the new head of US Pacific Command.

"North Korea needs to be watched very closely," Willard told reporters.

Leader Kim Jong-Il is thought to be grooming his youngest son as successor.

In his speech, Gates said the North's conventional military was still a threat but its "ability to launch another conventional ground invasion is much degraded from even a decade or two ago."

More worrying was its efforts to sell nuclear and missile technology abroad, the defense secretary said.

"I also think North Korea is a serious proliferation threat. Everything they make they seem to be willing to sell," he said in a question-and-answer session with the troops.

Gates, who arrived from Japan, will hold talks with Defence Minister Kim Tae-Young Thursday before leaving for a NATO meeting in Slovakia.

Apart from North Korea, US officials said South Korea's preparations to assume wartime operational command over its troops in 2012 will be on the agenda.

Seoul relinquished command over its troops to the United States during the 1950-53 Korean War and regained peacetime control in 1994.

Gates said the transfer illustrated a promising change in Washington's relationship with Seoul.

"These shifts have reflected the evolution of the US role -- from protector of a war-torn land to the role of a full partner with one of the world's most dynamic economies and capable militaries."