The air manoeuvres, which involved some 20 warplanes, come a day after the allies fired eight ballistic missiles into the Sea of Japan in the wake of Pyongyang's Sunday weapons tests.
On Sunday, North Korea fired eight short range ballistic missiles from four different locations in the space of 30 minutes -- one of its largest-ever tests, analysts said.
The nuclear-armed country has carried out a blitz of sanctions-busting weapons tests this year, including firing an intercontinental ballistic missile at full range for the first time since 2017.
US and South Korean officials have been warning for weeks that Kim Jong Un's regime is preparing to carry out a fresh nuclear test.
US deputy secretary of state Wendy Sherman, in Seoul on a three-day visit, warned there would be a "swift and forceful" response were Pyongyang to go ahead with what would be its seventh such test.
"Any nuclear test would be in complete violation of UN Security Council resolutions," she told reporters after meeting with her South Korean counterpart.
The entire world will respond in a "strong and clear manner," she said, adding: "We are prepared."
Sherman said the United States was still willing to engage in dialogue with the North, saying that Washington "harbours no hostile intent towards" Pyongyang.
Tuesday's air manoeuvres are the third such joint show of force by the allies under South Korea's hawkish new president Yoon Suk-yeol, who has vowed a tougher stance against Pyongyang.