North Korea prepared for war by firing rockets and torpedoes at mock enemy warships, the Daily Mail in London reported.
It was believed to be the nation's "largest ever" live-fire artillery drill, the according to the newspaper.
The exercise involved over 300 artillery guns. The large-caliber self-propelled guns were lined up along the coast and fired on orders from North Korean leader Kim Jong Un, CNN said. The news network attributed the information to the state news agency KCNA.
The show of force came as North Korea marked the 85th anniversary of the North Korean army and amid rising tensions with the United States, according to CNN.
"Our military is closely monitoring the North Korean military's movements," said a statement from the South Korean Joint Chiefs of Staff.
Experts say the exercise was likely designed to send a message to South Korea.
"So it is a sign that if the North is provoked or there is preemptive action, then a lot of Seoul and its suburbs would be within artillery range of the North."
Alex Neill, a senior fellow at the International Institute for Strategic Studies Asia told CNN.
Neill warned nuclear and chemical weapons could also be delivered by artillery strikes, CNN noted.
"As (North Korea has) one of the largest stockpiles of chemical weapons, again artillery can be used for that," he said. "Sarin gas doesn't have to be dropped."
Meanwhile, South Korea conducted military live-fire drills near the border following the North's exercise, according to the Daily Mail.
The South also announced that key parts of a U.S. missile defense system have now been installed, the newspaper said. The announcement came as a U.S. guided-missile submarine docked in South Korea and the USS Carl Vinson aircraft carrier neared the Korean peninsula, the Mail reported.