North Korea says it will continue to fire heavy weapons into waters along a maritime border it has disputed for decades.  North and South Korea traded warning shots in the area, just days after Pyongyang unilaterally declared it off limits to ships.

South Korea's Yonhap news agency reported North Korean fired a second artillery barrage into its coastal waters Wednesday, just hours after the first salvo.

North Korea's army warned it will continue to fire live ammunition in its coastal waters as part of what it calls an annual military drill.

South Korean officials say that Wednesday morning the North fired about 30 artillery shells into the ocean west of the Korean peninsula.  The South then fired about 100 shells into the air from a marine base on a nearby island as a warning to the North. 

South Korean officials say the North's artillery shells did not reach southern territorial waters. No casualties have been reported.

South Korean Unification Ministry spokeswoman, Lee Jeong-joo, says Seoul is paying close attention to the day's events.

She says the government is carefully studying military movements in North Korea, as well as the North's intentions, to determine an appropriate response.

South Korean President Lee Myung-bak is visiting India, but his chief security ministers convened an emergency meeting in his absence.  Defense Ministry spokesman Won Tae-jae says the South's forces are on high alert.

He says the South sent a written message to the North expressing "serious concerns" about its threatening actions, and its creation of unnecessary tension.

Wednesday's exchange took place along a maritime border mandated by U.S.-led United Nations forces in 1953 when an armistice halted fighting in the Korean War.

North Korea considers the border invalid.  The two Koreas fought deadly naval skirmishes in the area in 1999 and 2002.  South Korea also says its navy badly damaged a North Korean patrol boat that opened fire in November.  

This week, the North unilaterally declared a "no sail zone" for vessels near its coast, which many regional security analysts see as a precursor to artillery or missile tests.

Professor Kim Yong-hyun, a North Korea scholar at Seoul's Donguk University, thinks the provocations are meant to send a signal to the United States.

He says before the expected resumption of six-nation nuclear disarmament talks, North Korea is trying to stress the urgency of concluding a permanent peace treaty for the peninsula.

A peace treaty is one of the goals in an agreement to give up nuclear weapons that North Korea signed in 2005, then rejected.

Professor Kim says North Korea showed caution in firing artillery, not missiles, and ensuring the shells landed on the northern side of the maritime border.  He says that is a signal North Korea is still interested in international cooperation.

Wednesday's maritime exchange comes less than a week before U.S. Assistant Secretary of State Kurt Campbell visits Seoul to consult on North Korea and other security issues.