The annual drill takes place amid rising tensions in the region, which saw North Korea test-launch its most advanced Intercontinental Ballistic Missile (ICBM), allegedly capable of hitting anywhere on the US mainland, a week earlier.
More than 200 aircrafts are involved in the five-day war games, including six F-22 Raptors and six F-35 stealth fighter jets. About 12,000 US military personnel are taking part.
It is the first time that so many Raptors have been deployed simultaneously in an exercise in South Korea.
Al Jazeera's Kathy Novak, reporting from outside Osan Air Base in the South Korean city of Pyeongtaek, said the drills were "part-training and part-show of force against North Korea".
"They take place every year and always anger North Korea, which calls them a 'rehearsal for invasion'," Novak said.
"Before these drills went ahead, the North Korean foreign minister said that the US was begging for nuclear war by staging an extremely dangerous nuclear gamble on the Korean peninsula," she added.
The joint war games known as Vigilant air combat exercise (ACE) are "aimed at enhancing the all-weather, day and night combined air power operation capabilities of South Korea and the US," South Korea's defence ministry told local news agency Yonhap News.
They followed reports that South Korea's military created a "decapitation unit" with the aim of neutralising North Korea's leadership and destroying nuclear and missile facilities if war breaks out.
On Sunday, US National Security Adviser HR McMaster said Trump was prepared to "take care" of North Korea over their nuclear weapons programme.
"If necessary, the president and the United States will have to take care of it because he has said he's not going to allow this murderous, rogue regime to threaten the United States with the most destructive weapons on the planet," he told Fox News.