SEOUL, South Korea — North Korea warned it will aggressively defend itself in disputed waters where a bloody naval clash with rival South Korea took place last week, though the North tempered its rhetoric by indicating it wants better relations with Seoul and Washington.

The exchange of fire off the western coast was the first in seven years and came ahead of a trip to Seoul by President Barack Obama. A senior South Korean military officer said one North Korean officer died and three others were wounded. South Korea suffered no casualties.

North Korea's warning Friday — issued by a high-ranking military officer — follows vows to punish South Korea carried in North Korean state newspapers Thursday. The military also repeated its demand that South Korea apologize for Tuesday's skirmish.

The divided Koreas have long been at odds over their western sea border and fought battles in the area in 1999 and 2002. The North insists a line imposed by the U.N. command at the end of the 1950-53 Korean War be redrawn farther south, a demand rejected by South Korea.

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

It said South Korea would be forced to pay a heavy price for the firefight, though it did not elaborate.

Despite those strong words, the message also indicated North Korea's wish to continue a thaw in relations with South Korea and the United States.

It blamed what it described as "conservative forces" in South Korea as well as its military for trying to spoil moves toward detente on the Korean peninsula.

According to the North's official Korean Central News Agency, the warning was sent to South Korea by a military officer who heads the military delegation that occasionally holds talks with South Korea's military.

Both Koreas have blamed the other for provoking the skirmish.

In Washington, the State Department told Pyongyang to avoid behavior that raises tensions.

"We would urge North Korea to refrain from that kind of bellicose rhetoric and, in general, avoid any kind of provocative actions that would further inflame the tension in the region," said spokesman Ian Kelly.

South Korean officials have largely shrugged off the North's threats, saying they can deter any military moves off the coast. The country's 680,000-member military has been on guard following the skirmish, though officials said they have seen no evidence of unusual North Korean moves.