The United States and South Korea have kicked off their annual military exercise, known as Ulchi Freedom Guardian.
The wargames use computer simulations to prepare for conflict with nuclear-capable North Korea.
Troops from seven allied nations, including Britain, Canada and Australia, are also taking part.
South Korean President Moon Jae-in denied Pyongyang’s accusation that the drills are a rehearsal for an invasion.
“This year’s Ulchi exercise is to check the defensive postures of our civilians, government and military to secure the lives and safety of our people”, Moon said at a cabinet meeting. “It is an annual exercise of a defensive nature, and there is no intention of heightening military tensions on the Korean Peninsula”.
Peace activists in Seoul protested the decision to go ahead with the drill, concerned it could trigger retaliation by North Korea. Pyongyang last week appeared to back down from a threat to launch a missile strike on the US territory of Guam.
China has also urged Washington to scrap the ten-day-long exercise.