The situation on the Korean peninsula grew tense again on Monday, when North and South Korea exchanged fire over the maritime border in the Yellow Sea. Pyongyang said it was conducting tests, which lead to the two countries launching 800 shells.

The exchange began with a fax from Kim Jong Un’s regime to Seoul, reports NBC News. It was followed by 500 shells from the North, the South Korean defense ministry said.

“We believe the North's maritime firing is a planned provocation and an attempt to test our military's determination to defend the Northern Limit Line and to get an upper hand in South-North relations,” South Korean Defence Ministry spokesman Kim Min-seok told Reuters.

The North called the exercises a response to the UN’s condemnation of its missile launches last week. It also accused the South of “gangster-like” in an incident with a fishing boat over the weekend. The South said that it had just let the boat go back after it drifted into South waters.

Nevertheless, the North launched 500 shells, about 100 of which landed in South Korean waters. The South responded by firing another 300 shells into the North’s waters.

Tensions have reached new heights recently as the North’s rhetoric has become increasingly combative. The two countries are still technically at war, since the 1950-53 Korean War only ended in an armistice.