SEOUL, South Korea (AFP) — South Korea and the United States began their first combined naval exercise near the peninsula in five years on Monday, a day after Pyongyang conducted a ballistic missile launch.

Washington is Seoul's key security ally and stations about 28,500 troops in South Korea to protect it from the nuclear-armed North.

South Korea's hawkish President Yoon Suk-yeol, who took office in May, has vowed to beef up joint military exercises with the US, after years of failed diplomacy with North Korea under his predecessor.

"This exercise was prepared to demonstrate the strong will of the South Korea-US alliance to respond to North Korean provocations," the South's navy said in a statement.

The four-day exercise on South Korea's east coast will involve more than 20 vessels and an assortment of aircraft, which will conduct drills on anti-ship and anti-submarine warfare operations, tactical maneuvers and other maritime operations, it added.