"Foal Eagle is being reorganized a bit to keep it at a level that will not be harmful to diplomacy," Mattis said, adding that it would be "reduced in scope."

US and South Korean forces have been training together for years, and routinely rehearse everything from beach landings to an invasion from the North and even "decapitation" strikes targeting the North Korean regime.

But personal insults and threats of war between US President Donald Trump and North Korean leader Kim Jong Un were replaced last year by a rapid diplomatic rapprochement.

The thaw culminated in a historic summit in Singapore in June, where the leaders signed a vaguely-worded document on denuclearization of the peninsula.

The US and South Korea 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.

In September, the then nominee to head US and UN forces in South Korea said the pause in drills had been a "prudent risk" to help facilitate a detente on the peninsula.

But there "was certainly a degradation in the readiness of the force, for the combined forces," General Robert Abrams told the Senate Armed Services Committee at his confirmation hearing.

Abrams went on to say that the continued suspension of the drills risked a further erosion in "readiness and capability and interoperability of the combined forces," though he noted officials were working to minimize issues by running smaller scale staff exercises.