SEOUL, SOUTH KOREA - North and South Korea say they have exchanged warning shots near a disputed area off their west coast, the latest indication of heightened military tensions on the Korean Peninsula.

South Korea's military said it fired warning shots at a North Korean merchant ship that crossed the Northern Limit Line, the de facto Korean sea border, at 3:42 a.m. Monday, local time.

About 90 minutes later, North Korea fired 10 shells from a multiple rocket launcher system in response, according to a North Korean military statement posted on state media.

The North Korean ship crossed the NLL near the South Korean island of Baengnyeong but returned north after the warning, South Korea's military said.

North Korea does not recognize the NLL and its ships sometimes cross the border, resulting in tense military incidents.

Neither side reported any clashes in Monday's incident.

Tensions have grown as North Korea conducts an unprecedented barrage of missile tests this year. There are also concerns the North could soon carry out its seventh nuclear test since 2006.

In recent weeks, North Korea has conducted provocative artillery shelling near the inter-Korean sea border and boasted of its ability to conduct strikes with tactical nuclear weapons.

In response, the United States and its ally, South Korea, have increased displays of military strength and ramped up joint defense exercises that had been scaled back for years.

The moves have raised concerns that the two Koreas, still technically in a state of war after their 1950s conflict ended only in a truce, are entering a particularly dangerous phase.

North and South Korea held historic talks in 2018, leading to several agreements that reduced military tensions in the shared border areas. But North Korea, frustrated by the lack of progress in parallel nuclear talks with the United States, soon walked away from both sets of negotiations.

This year, North Korea has tested a record-high 41 ballistic missiles. It also has passed a new law underscoring its status as a nuclear weapons state and declaring it will never abandon its nuclear arsenal.