North Korea fired about 110 rounds of artillery on Monday near its disputed sea border with South Korea, the South's military said, amid high tension over the deadly sinking of a South Korean warship blamed on North Korea.

The firing came shortly after South Korea ended a five-day naval drills off the west coast, that the North called a rehearsal for an invasion.

All the artillery shells harmlessly landed into the North's waters and caused no damage to the South, a South Korean Joint Chief of Staff officer said on condition of anonymity, citing department policy.

Relations between the rival Koreas are at their lowest point in recent years following the March sinking of the South Korean warship that an international investigation blamed on a North Korean torpedo attack.

Tension deepened last week when South Korea launched large-scale naval training in response to the sinking, prompting its communist neighbor to warn it would counter the maneuvers with a "strong physical retaliation."

On Sunday, North Korean authorities seized four South Korean and three Chinese fishermen aboard a 41-ton South Korean fishing boat for an alleged violation of the North's exclusive economic zone. South Korea has demanded the North quickly release them.

The South Korean naval drills, which were in their final day Monday, followed large-scale joint military exercises with the US last month off the east coast.

They included exercises near the disputed inter-Korean sea border where the warship Cheonan exploded and sank, killing 46 sailors. The area is also where the navies of the rival Koreas fought three bloody gunbattles since 1999.

The two Koreas are still technically at war because their 1950-53 Korean War ended with an armistice, not a peace treaty. The US stations 28,500 troops in South Korea to deter any potential aggression from North Korea.