South Korea’s military said its soldiers fired warning shots at North Korean troops who crossed the demarcation line between the two countries.
Around 10 North Korean soldiers violated the military line at 5pm local time on Tuesday, but returned after it made warning broadcasts and fired warning shots, South Korea’s military said, adding that it is closely monitoring its neighbour’s activities.
Since re-entering the White House, US President Donald Trump has said he would reach out to North Korean leader Kim Jong Un again to revive diplomatic efforts begun during his first term.
The secretive regime has not responded to Mr Trump’s remarks, but recently claimed US hostilities against it had deepened since the inauguration.
Last month, Mr Kim’s sister threatened the Trump administration with retaliatory action for stepping up “provocations” with the deployment of a US aircraft carrier to South Korea.
There is no clear motive for Tuesday’s border crossing by North Korean soldiers.
Tensions between the two countries have been running high as North Korean leader Kim Jong Un continues to order missile tests and align with Russia over President Vladimir Putin’s war on Ukraine.
North Korean special operations units are among the thousands of troops that Pyongyang has sent to Russia to fight in the Ukraine war, according to South Korea.
In January, Pyongyang tested a hypersonic missile of intermediate range, in a sign of its determination to continue its weapons development programme.
Bloodshed and violent confrontations have occasionally occurred at the Koreas’ heavily fortified border, called the Demilitarised Zone, or DMZ.
But when North Korean troops briefly violated the border in June last year, it didn’t escalate as South Korean officials realised they were carrying construction tools and decided the incursion was accidental.
The 155-mile (249km)-long, 2.5-mile (4km)-wide DMZ is the world’s most heavily armed border.
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Around two million mines are inside and near the border, which is also guarded by barbed wire fences, tank traps and combat troops on both sides.
It’s a legacy of the 1950-53 Korean War, which ended with an armistice, not a peace treaty.
The incursion comes as South Korea copes with a leadership vacuum after the ousting of President Yoon Suk Yeol last week over his ill-fated imposition of martial law.