SEOUL >> South Korean marines returned fire today after North Korea launched artillery shells into waters near the disputed maritime line that separates the two rivals, South Korean defense officials said.

The three North Korean shells fired near the Northern Limit Line in the Yellow Sea prompted the South to fire three shells back, Defense Ministry spokesman Kim Min-seok said. Another ministry official, who refused to be named because of office policy, said North Korea fired more rounds later in the day and South Korea responded. He could not immediately say how many rounds were fired by both sides.

Both sides’ shells landed in the water, and there were no reports of casualties.

South Korean forces have been on high alert in the area since a North Korean artillery attack killed four people in November on South Korea’s Yeonpyeong island. Today’s artillery exchange, which happened in hazy weather, was near that island.

The firing follows a recent easing of animosity between the Koreas and could be a warning about joint U.S.-South Korean military drills set for next week. Last month, a senior North Korean diplomat met with U.S. officials in New York to negotiate ways to restart long-stalled international talks aimed at persuading the North to abandon its nuclear weapons aspirations. The meeting came after the Koreas’ nuclear envoys held cordial talks during a regional security forum in Indonesia.

The official who spoke on condition of anonymity said South Korean forces stepped up their monitoring of the North after today’s artillery exchange. South Korean marines on Yeonpyeong returned fire after North Korea fired from one of its front-line islands, the official said.

The North’s shelling took place unexpectedly, the official said, and neither side was conducting firing drills at the time. The South Korean military has yet to determine the motive behind the North’s shelling, the official said.

Neither the North’s government nor its official news agency immediately commented on the shelling.

Violence often erupts in the contested slice of sea. Boats routinely jostle for position during crab-catching season, and three deadly naval clashes since 1999 have taken a few dozen lives.

Kim said one North Korean artillery shell is believed to have fallen south of the maritime line, citing a preliminary analysis of the trajectory of the shell.

The line separating the countries was drawn by the U.S.-led U.N. Command without Pyongyang’s consent at the close of the 1950-53 Korean War, which ended with a truce, not a peace treaty, leaving the peninsula still technically in a state of war. The line is still a fierce point of dispute.

North Korea argues that the line should run farther south. Seoul believes accepting such a line would endanger fishing around five South Korean islands and hamper access to its port at Incheon.

The November attack marked a new level of hostility along the contested line. Two South Korean civilians and two marines died, and many houses were gutted in the shelling.

Baek Seung-joo, a military analyst at the state-run Korea Institute for Defense Analyses in South Korea, said the North appears to be rattling its sabers ahead of annual U.S.-South Korean military exercises planned for next week.

North Korea routinely denounces Seoul and Washington for such drills, calling them precursors to an invasion. The impoverished North faces heavy economic pressure when it is forced to mobilize its own military to counter South Korean drills.

On Monday, a North Korean military spokesman released an open letter that called the joint exercises “hideous provocations.” He warned that the North has access to a “nuclear deterrent powerful enough to protect” itself.

The North has conducted two nuclear tests since 2006.

Baek also said the North appears to be keeping tension alive in an effort to unite its own people, even as it moves to restore dialogue with the outside world.

Meanwhile, South Korea’s Red Cross said in a statement that it has sent Pyongyang a list of items meant to help North Korea recover from recent flooding and heavy rain. The items included baby food, cookies and instant noodles.

The North has yet to accept the aid offer. Last week, the North’s Red Cross asked the South to send concrete as well. The South refused. Seoul worries such material may be used for military purposes.