North Korea said Wednesday it had fired artillery shells off its east and west coasts the previous night to "send a serious warning" to South Korea, according to the country's official media, with Seoul saying that the firing resumed Wednesday afternoon.

The South Korean military's Joint Chiefs of Staff said Pyongyang fired a total of some 250 artillery rounds from 10 p.m. Tuesday toward the Yellow Sea and the Sea of Japan, with an additional 100 shots fired from around 12:30 p.m. Wednesday into waters off the west coast.

A spokesman for the General Staff of the Korean People's Army in North Korea said in a statement Tuesday's artillery fire was "a powerful military countermeasure" to South Korea's firing of dozens of rounds from multiple rocket launchers in an area along the Military Demarcation Line, the Korean Central News Agency said.

The General Staff ordered eastern and western frontline units of the KPA to "open a threatening, warning fire once again" in response to South Korea's fresh launch Wednesday morning of over 10 shells from multiple rocket launchers in the frontline zone, KCNA said.

"The enemy's war drill" against the North is "going on in a frantic manner," the KCNA quoted the unnamed spokesman as saying. He called on Seoul to "immediately stop the reckless and inciting provocations which are escalating military tensions in the forefront area."

The South Korean military began Monday a field training exercise involving U.S. forces in the country. The JCS has urged Pyongyang to immediately halt its actions.

None of the shots fired Tuesday by the North fell into the territorial waters of the South, the JCS added.

The latest barrage comes after the North fired artillery shells into coastal waters last Friday, which was also purported to be a "countermeasure" to South Korean artillery fire earlier that day.

North Korea has recently been test-firing ballistic missiles at an increased rate, and speculation has been growing that it could conduct what would be its seventh nuclear test and first since September 2017.