US defence chief Mattis assures South Korea of 'ironclad' commitment

US Defense Secretary Jim Mattis on Thursday assured South Korea of an "ironclad" commitment to its security, including keeping US troop levels unaltered, even as diplomats seek an agreement with North Korea on denuclearisation.

Mattis, during a brief visit to Seoul, defended US President Donald Trump's decision this month to halt "war games" with South Korea, saying it would increase the opportunity for diplomats to negotiate.

But he added that US and South Korean forces would remain "united, vigilant and ready."

Critics of Trump's decision on the joint military exercises say it is a major concession to North Korea that, if prolonged during what are expected to be lengthy negotiations, risk eroding the readiness of US and South Korean forces.

North Korea has long sought a suspension of the drills.

Trump has also drawn criticism from national security analysts for an agreement that emerged from his June 12 summit with Kim Jong-un – leader of the Democratic People's Republic of Korea (DPRK) – that had few details on how Pyongyang would surrender its nuclear weapons and ballistic missiles.

"US commitment to the Republic of Korea remains ironclad and the US will continue to use the full range of diplomatic and military capabilities to uphold this commitment," Mattis said in brief remarks ahead of talks at the South Korean defence ministry.

"And this includes maintaining the current US force levels on the Korean peninsula."

The United States has about 28,500 troops in South Korea.

"Fight tonight"

Still, Mattis has yet to explain how the United States will maintain readiness of US-South Korean forces, who have long prided themselves on the ability of being able to "fight tonight" if needed.

Washington and Seoul have already suspended planning for the upcoming Freedom Guardian exercise, which last year involved 17,500 American troops and more than 50,000 South Korean troops.

"The recent decision to suspend the Freedom Guardian exercise creates increased opportunity for our diplomats to negotiate, increasing prospects for a peaceful solution on the Korean peninsula," Mattis said, standing alongside his South Korean counterpart.

"At the same time, the US and [South Korean] forces remain united, vigilant and ready to defend against any challenge."