SEOUL, South Korea – South Korea began naval drills off its west coast Thursday to strengthen its ability to counter any North Korean provocation despite Pyongyang's latest threat to retaliate over the war games.

The drills, which follow joint military exercises with the U.S. last month off the east coast, are set to run through Monday and will include exercises in areas near the disputed western sea border with North Korea. They involve about 4,500 South Korean troops, more than 20 ships and submarines, and about 50 aircraft, according to the Joint Chiefs of Staff.

Both sets of maneuvers came in response to the deadly March sinking of a South Korean warship blamed on North Korea. Forty-six South Korean sailors were killed. The U.S. and South Korea said the drills were to warn the North that further provocations will not be tolerated.

A five-nation team of investigators concluded in May that a North Korean torpedo sank the 1,200-ton Cheonan warship in late March near the Koreas' western maritime border.

North Korea, which denies any involvement in the sinking, warned Tuesday it will "counter the reckless naval firing projected by the group of traitors with strong physical retaliation."

The North had also threatened to respond to the South Korea-U.S. military exercises with "nuclear deterrence" but South Korean military officials said there has been no sign of unusual North Korean military activity.

North Korea routinely issues such threats, especially when the South holds joint military drills with the U.S. Pyongyang sees the exercises as a rehearsal for an invasion. The U.S. has 28,500 troops in South Korea to protect the longtime Asian ally.

The North's military also warned Tuesday civilian ships to stay away from the sea border — the scene of deadly skirmishes between the two sides in 1999, 2002 and last year.

The western maritime boundary has long been a flash point between the two Koreas because the North does not recognize the border unilaterally drawn by the United Nations at the close of the 1950-53 Korean War.

The Korean peninsula remains in a technical state of war because the Korean War ended in a truce, not a peace treaty.

South Korea was closely monitoring North Korea's military but spotted no unusual activity Thursday, South Korea's Joint Chiefs of Staff said.