SEOUL, South Korea – The rival Koreas exchanged warning shots near their disputed western sea border on Monday, their militaries said, amid heightened tensions over North Korea’s recent weapons tests.

South Korea’s Joint Chiefs of Staff said in a statement that its navy had fired warning shots to repel a North Korean merchant ship that it said had violated its maritime boundary early Monday.

North Korea’s military said it responded by firing 10 artillery shells as a warning to South Korea. He accused a South Korean navy ship of invading North Korean waters.

There were no reports of clashes between the Koreans.

The ill-marked maritime boundary off the west coast of the Korean Peninsula is a source of long-standing hostility between the Korean countries. It has been the scene of several bloody inter-Korean naval skirmishes and violence in recent years, including two attacks in 2010 that killed 50 South Koreans.

In recent weeks, North Korea has carried out a series of weapons tests in response to what it calls provocative military exercises between South Korea and the United States. Since September 25, North Korea has launched 15 missiles and hundreds of artillery shells towards the sea.

Seoul and Washington regularly hold military exercises to maintain their readiness against potential North Korean aggression. Allies say their exercises are defensive in nature, but North Korea sees them as a rehearsal for an invasion.

The South Korean military is conducting its annual field exercises, which will conclude this Friday. An unspecified number of American military personnel are taking part in this year’s exercises.

Some experts say North Korea’s recent weapons tests are also aimed at bolstering its arsenals to increase its leverage in future negotiations with its rivals.