South Korea and the United States held anti-submarine drills on Tuesday, the third day of the four-day joint military drills.

The two allies conducted drills involving naval guns, torpedoes and artillery to defeat a submarine intruding into waters off the Korean peninsula. Fighter jets and helicopters engaged in bombing practice, according to local news reports.

Code-named "Invincible Spirit," the four-day naval and air exercises involving 20 ships and submarines, 200 aircraft and 8, 000 troops are part of a slew of punitive measures by Seoul and Washington against Pyongyang after a South Korea-led multinational probe team concluded that the Democratic People's Republic of Korea (DPRK) torpedoed a South Korean warship in March, which took 46 lives of South Korean sailors.

Tuesday's drills were aimed at detecting and attacking DPRK vessels infiltrating into South Korean waters to prevent recurrences of the March 26 attack on the corvette Cheonan.

Pyongyang, which has denied its involvement in the sinking, recently threatened a "retaliatory sacred war" against the show of joint military prowess. China has also expressed its concern over the drills near the country, though officials from South Korea and the U.S. said the drills are of defensive nature.

Seoul and Washington are expected to stage another round of large-scale naval drills in September in waters off the west coast of the Peninsula, where the Cheonan sank near a contentious maritime border between the two Koreas.