North Korea warned Friday that it will mercilessly defend itself in disputed waters where a bloody naval clash with rival South Korea took place this week, further raising tensions ahead of a visit to Seoul by President Barack Obama.

The North's military issued the warning in a message sent to South Korea, according to Pyongyang's official Korean Central News Agency said. The message was sent by a high-ranking officer who heads the North Korean military delegation that occasionally holds talks with South Korea's military.

Two state newspapers issued a similar warning Thursday, though analysts played down the threat as they were not issued by the military itself.

The military also repeated a demand that South Korea apologize over the clash off their western coast Tuesday that a senior South Korean military officer said left one North Korean officer dead and three others wounded.

North Korea also reiterated it would not honor the Northern Limit Line, a de facto western sea border drawn up by the U.N. command at the end of the 1950-53 Korean War.

The divided Koreas have long been at odds over the western sea border. The North has insisted the line be redrawn farther south, a demand rejected by South Korea.

North Korea's military said it "will take merciless military measures to defend" its interpretation of the border "from this moment."

South Korea Defense Ministry said its position on the sea border remains unchanged. The Joint Chiefs of Staff had no comment.

The North's military also said the South will be forced to pay a heavy price over what it claims was South Korea's intentional provocation.

The two sides have accused the other of provoking the two-minute battle on Tuesday.

South Korea suffered no casualties.

The two Koreas have remained technically at war since the Korean War ended with an armistice, not a peace treaty. The U.S., which has never had diplomatic relations with North Korea, stations 28,500 troops in South Korea to deter potential North Korean aggression.

Obama is due to arrive in Seoul Wednesday as part of a regional trip.