The exercises, renamed Ulchi Freedom Shield this year and scheduled to end on September 1, came after South Korean President Yoon Suk-yeol - who took office in May - vowed to "normalise" the combined exercises and boost deterrence against the North.
The drills had been scaled back in recent years because of COVID-19 and as Yoon's predecessor Moon Jae-in sought to restart talks with Pyongyang, which has described the exercises as a rehearsal for invasion.
North Korea fired two cruise missiles from the west coast town of Onchon last week, after South Korea and the United States kicked off preliminary training for the exercises.
North Korea has conducted missile tests at an unprecedented pace this year and is ready to conduct its seventh nuclear test at any time, Seoul officials said.
Yoon has said his government is willing to provide economic aid if Pyongyang takes steps toward denuclearisation, but North Korea has rebuffed his offer and openly criticised him.
Seoul's defence ministry has said the allies would stage 11 field training programs, including one at brigade-level - involving thousands of soldiers - this summer.
To better counter North Korea's growing missile threats targeting the South's capital, the ministry said it would improve missile detection capabilities and push for an early deployment of a new interceptor system.
The United States, South Korea and Japan participated in a recent ballistic missile defence exercise off Hawaii's coast, the first such drills since 2017 when relations between Seoul and Tokyo hit their lowest point in years.
