The US played a key role in defending the South after the North invaded in 1950, triggering the Korean War, and even now stations 28,500 troops in the country, a treaty ally, to protect it from its neighbour.

US President Donald Trump and North Korean leader Kim Jong Un traded personal insults and threats of war last year, only for fears of conflict to be replaced by a rapid diplomatic rapprochement.

But as progress has slowed in recent months there has been a growing uneasiness between the allies, with the US firm on sanctions against Pyongyang while Seoul is seeking to relax measures on its neighbour.

"In this place we have never succeeded by going alone," General Vincent Brooks said in his last act as the commander of US Forces Korea, the UN Command and the South Korea-US Combined Forces Command.

"Our fears and our concerns should rise if we become inclined to go our own way."

On the campaign trail US President Donald Trump raised doubts about the continued presence of US troops in South Korea.

This week the chairman of the US Joint Chiefs of Staff said the US would need to make "some changes to the military posture on the peninsula" over time if talks with Pyongyang progress.

Over the 65 years of the alliance, Brooks told a change-of-command ceremony at Camp Humphreys in Pyeongtaek, "we grew stronger under the tests and strains that confronted us, contrary to the predictions of cracks and fissures".

"Let this be a lesson to all in the alliance," he added.

Brooks, who took up his post in April 2016, has described his time in the South as "a rollercoaster ride".

He previously said he was given no prior indication that Trump, after his summit with North Korean leader Kim Jong Un in June, would announce the cancellation of "very provocative" and expensive joint military drills with the South.

The allies have since suspended most of their major joint exercises, including the Ulchi Freedom Guardian in August and the Vigilant Ace air force training initially slated for next month.

His successor General Robert B. Abrams told his Senate confirmation hearing there "was certainly a degradation in the readiness of the force, for the combined forces" as a result of the pause in drills.

At Thursday's ceremony Abrams -- whose father was a former Army Chief of Staff for whom the M1 Abrams tank is named -- vowed to continue Washington's "ironclad relationship" with Seoul.

The military would maintain its capability so "we cannot only deter but defeat external threats if we are called to do so", he said.