WASHINGTON—Defense Secretary Jim Mattis said the U.S. doesn’t plan to suspend any more military exercises with South Korea, as negotiations continue with the North, but added that no decision had yet been made on conducting another large maneuver next year.

The Pentagon called off a major military exercise with South Korea in a show of good faith following President Trump’s meeting with North Korean leader Kim Jong Un in Singapore in June. That included an exercise known as Freedom Guardian, in which more than 50,000 U.S....