By Soo-hyang Choi and Hyonhee Shin

SEOUL (Reuters) -North and South Korea exchanged warning shots off the west coast on Monday, accusing each other of breaching their western maritime border amid heightened military tension.

The South's Joint Chiefs of Staff (JCS) said it broadcast warnings and fired warning shots to see off a North Korean merchant vessel that crossed the Northern Limit Line (NLL), the de facto sea boundary, at around 3:40 a.m. (1840 GMT Sunday).

The North's military said it fired 10 artillery shells after a South Korean navy ship violated the NLL and fired warning shots "on the pretext of tracking down an unidentified ship," according to state media.

"We ordered initial countermeasures to strongly expel the enemy warship by firing 10 shells of multiple rocket launchers near the waters where the enemy movement occurred," a spokesperson for the General Staff of the North's Korean People's Army said in a statement carried by the official KCNA news agency.

The JCS said it had conducted a "normal operation" over the border intrusion, and called the North's move a "provocation" and a violation of a 2018 bilateral military pact banning "hostile acts" in the border areas.

"We once again urge North Korea to immediately cease consistent provocations and accusations which harm the peace and stability of the Korean peninsula as well as the international community," the JCS said in a statement.

The latest exchange of fire came amid simmering tension, with the North carrying out weapons tests at an unprecedented pace this year.

In recent weeks, North Korea launched short-range ballistic missiles and hundreds of artillery rounds off its east and west coasts on several occasions in protest over the South's military activities.

South Korea's troops kicked off their annual Hoguk defence drills last week, designed to run until Oct. 28 and boost their own and combined ability with the United States to counter the North's nuclear and missile threats.

Pyongyang has angrily reacted to the drills, calling them provocations and threatening countermeasures. Seoul and Washington say their exercises are defensive and aimed at deterring the North.