The South Korean military fired warning shots at a North Korean ship before dawn on Monday, as it believed the ship had crossed the opponents’ disputed sea border, prompting the North to return the warning fire, state and local media reported.

A North Korean merchant ship crossed the so-called northern border line at 3:42 a.m. but retreated north after the south’s navy fired warning shots, South Korea’s Joint Chiefs of Staff said, according to Yonhap news agency.

The Korean People’s Army of Pyongyang claimed a South Korean military ship “entered” the de facto border by 2.5 to 5 kilometers (1.5 to 3 miles) a few minutes later and the KPA had ten warning rounds from the country’s west coast fired.

KPA “Coastal defense units on the western front … took an initial countermeasure to forcefully drive off the enemy warship by firing 10 shells from multiple rocket launchers at 5:15 a.m. at territorial waters where enemy sea movements were detected,” a KPA said a spokesman for the General Staff in a statement.

“The KPA General Staff once again sends a serious warning to the enemies who have carried out a naval invasion after such provocations as the recent artillery barrage and loudspeaker transmissions (a),” he added.

The maritime border is considered a flashpoint and has been the scene of several clashes between the two Koreas over the years.

Tensions have risen in recent weeks, with the North conducting multiple missile launches and artillery barrages that provoked South Korea and Japan and worried their western allies.

Pyongyang recently dramatically expanded its military drills as Seoul and Washington say North Korean leader Kim Jong Un is close to conducting his country’s seventh nuclear test.

Monday’s warning beacon replacement comes on the same day that US Assistant Secretary of State Wendy Sherman is due to hold tripartite talks with allies Japan and South Korea in Tokyo in a show of unity following the North’s numerous launches.

Earlier this month, North Korea fired a short-range ballistic missile, conducted multiple artillery barrages and flew warplanes near the southern border in another show of force.

Pyongyang has described the exercises as “tactical nuclear exercises”.