South Korea has begun naval drills off its western coast, aimed at strengthening its ability to counter military provocations from North Korea.
The exercises, held on Thursday, follow a joint military drill with the US last month off South Korea's eastern coast, and are set to run through to Monday.
The drills will include exercises in areas near the disputed western sea border, involving about 4,500 South Korean troops, more than 20 ships and submarines, and about 50 aircraft, according to the joint chiefs of staff.
Both exercises follow the deadly sinking of a South Korean warship in March that killed 46 sailors.
A five-nation team of investigators concluded in May that a North Korean torpedo sank the 1,200-tonne Cheonan frigate near the Koreas' western maritime border.
North Korea, which denies any involvement in the sinking, warned on Tuesday that
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-US military exercises with "nuclear deterrence'' but South Korean military officials said there has been no sign of unusual North Korean military activity.
North Korea, which sees the drills as a rehearsal for an invasion, routinely issues such threats, especially when the South holds joint military drills with the US.
The US has 28,500 troops in South Korea to protect its longtime ally.
North Korea's military also warned on Tuesday for 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 recognise 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.