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 United States, 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.

"Through this exercise, we will further improve the ability to conduct joint operations between the naval forces of the two countries," Kwak Kwang-sub, a senior South Korean naval officer, said in the statement.

The drills come a day after nuclear-armed Pyongyang conducted another ballistic missile launch, the latest in its record-breaking blitz of weapons tests this year.

North Korea is under multiple sets of international sanctions over its programmes to develop nuclear weapons and ballistic missiles.

Seoul had also detected signs the North was preparing to fire a Submarine Launched Ballistic Missile (SLBM), the president's office said Saturday, a weapon Pyongyang last tested in May.

Earlier this month, the North revised its nuclear law, enshrining a "first strike" doctrine and vowing never to give up its nukes.

The allies have long carried out joint exercises, which they insist are purely defensive. North Korea, however, sees them as rehearsals for an invasion.

Last month, the United States and South Korea staged their biggest combined military drills since 2018 -- the resumption of large-scale training sessions that had been scaled back due to Covid-19 and the period of diplomacy with Pyongyang.