North Korea and South Korea exchanged fire near a disputed border on Monday following a military exercise by the North that reportedly dropped hundreds of shells in South Korean waters.

Reuters reports that

The North had flagged its intentions to conduct the exercise in response to U.N. condemnation of last week's missile launches by Pyongyang and against what it says are threatening military drills in the South by U.S. forces.

From NBC News:

The North in recent weeks has increased threatening rhetoric and conducted a series of rocket and ballistic missile launches that are considered acts of protest against annual ongoing springtime military exercises by Seoul and Washington. The North calls the South Korea-U.S. drills a rehearsal for invasion; the allies say they're routine and defensive.

According to media reports, the North fired roughly 500 artillery shells, 100 of which landed in the South's territory. The South shot back with 300 shells.

"Some (North Korean) artillery fire landed in (the) southern part of Northern Limit Line but in the water," CNN quotes a South Korean Ministry of Defense spokesman as saying. "We counter-fired over the Northern Limit Line," the official said.

The Northern Limit Line (NLL) is a disputed border line between the North and South, and, as Tim Beal, author of North Korea: The Struggle Against American Power, wrote in 2012, the NLL "is a problem." Beal continued:

It is not accepted by North Korea and it cuts off their fishing boats from rich crab grounds. It has no legal basis, as has been admitted in private by the Americans, including Henry Kissinger. After the Yeonpyeong incident of 2010 many commentators, including the staunchly pro-American International Crisis Group, argued that it should be abandoned and replaced by a line acceptable to both North and South. The North’s Military Demarcation Line is an obvious choice, but a problem remains. The North does not contest the South’s control of the offshore islands and it has suggested the solution to be lanes of access

One important point to note is that the North considers the waters surrounding these islands, down as far as the MDL, to be theirs. This was a key issue in the 2010 confrontation.

No shots fired Monday targeted any land or military installations, an unnamed official with South Korea's Joint Chiefs of Staff told the Associated Press.

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