In June, after President Donald Trump held a historic summit with North Korean leader Kim Jong Un in Singapore, the United States said it would suspend "select" exercises with South Korea, including the large-scale Ulchi Freedom Guardian exercises slated for August.

"The Department of Defense suspended three individual military exercises in order to provide space for our diplomats to negotiate the verifiable, irreversible and complete denuclearization of the Korean peninsula," Mattis said in a statement.

"Our military posture has not changed since the conclusion of the Singapore summit and no decisions have been made about suspending any future exercises," he added.

At a press conference on Tuesday, Mattis said the Pentagon was not currently planning on halting future drills.

"We took the step to suspend several of the largest exercises as a good-faith measure coming out of the Singapore summit. We have no plans at this time to suspend any more exercises," he told Pentagon reporters.

In his statement on Wednesday, Mattis added that US and South Korean forces "maintain a high state of military readiness and vigilance in full support of a diplomatically-led effort to bring peace, prosperity and stability to the Korean peninsula."