South Korea's military fired warning shots to turn back a patrol boat from the North that had crossed the countries' de facto maritime border, Seoul said on Sunday.

The boat breached the Northern Limit Line on Saturday morning near South Korea's Baekryeong Island, the Joint Chiefs of Staff said in a statement.

"Our Navy's high-speed boat sent warning messages and conducted warning shots and immediately warded it off," it said.

"Our military is prepared against various provocations and keeping a decisive combat posture while closely monitoring the enemy's movements," the statement added.

During the operation, some South Korean crew members were being treated for injuries sustained when their vessel collided with a Chinese fishing boat.

The de facto maritime border -- which was never officially recognised by the North -- has long been a flashpoint between the two Koreas, which remain technically at war after the 1950-53 Korean War ended with an armistice rather than a peace treaty.

Last year the nuclear-armed North fired a ballistic missile across the maritime border, prompting Seoul to fire three missiles of its own in response, while also suspending flight routes and warning island residents in the area to shelter in bunkers.

Relations between the two Koreas are at one of their lowest points in years after Pyongyang declared itself an "irreversible" nuclear power last year, effectively ending the possibility of denuclearisation talks.

North Korean leader Kim Jong Un has doubled down on weapons testing, with Pyongyang firing several of its most powerful intercontinental ballistic missiles this year.

Washington and Seoul have ramped up defence cooperation in response, staging joint military exercises with advanced stealth jets and high-profile US strategic assets.

North Korea views such exercises as rehearsals for invasion and described them on Tuesday as "frantic" drills "simulating an all-out war against" Pyongyang. (AFP)