South Korea and the United States continued with their joint naval exercise Monday in waters off the east coast of the Korean peninsula, a day after the allies begun a four-day naval exercise in the face of growing tensions in the region.

The second day of the drills focused on anti-submarine exercises involving the U.S. aircraft carrier USS George Washington, aimed at detecting and defeating submarines intruding into the seas east of the peninsula.

The exercise consisted of simulated attacks on enemy submarines and live-fire drills by a squadron of fighter jets off the east coast, Seoul's Yonhap News Agency quoted Lee Bung-woo, a spokesman for the country's Joint Chiefs of Staff (JCS) as saying.

As part of Monday's drills, four F-22 Raptors, the world's most advanced powerful fighter jet flew over South Korea for the first time, according Yonhap.

On Tuesday, the two countries will fire naval guns and artillery at an abandoned submarine disguised as an enemy submarine, Lee was quoted as saying.

The drills will also involve firing underwater torpedoes, solidifying defense against sea and air intrusions and air-to-air refueling, Lee added.

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

The DPRK's National Defense Commission, the highest seat of power led by leader Kim Jong-il, threatened on Saturday to mount a nuclear response to counter the muscle flexing of Seoul and Washington, labeling the maneuvers "nothing but outright provocations."

Pyongyang has denied its role in the torpedo attack that sank the South Korean corvette Cheonan. The United Nations Security Council condemned the attack in a presidential statement but stopped short of directly blaming the DPRK.

Plans for the ongoing military exercises were announced last week by U.S. Defense Secretary Robert Gates and his South Korean counterpart, Defense Minister Kim Tae-young. They said the drills are meant to deliver a "clear sign" to the DPRK that provocations must be stopped.

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 46 South Korean sailors were killed in the sinking.

China expressed its opposition to U.S.-South Korean military drills at the Yellow Sea, which is a sea stripe between China and the Korean Peninsula, and expressed "deep concern" over the war games starting on Sunday.

"We sternly oppose activities that affect China's security by foreign military vessels or aircraft at the Yellow Sea or in China 's offshore waters," Foreign Ministry spokesman Qin Gang said Wednesday in a statement.

"We urge relevant parties to remain calm and exercise restraint and not do anything to exacerbate regional tensions," he said.