North Korea accused the South's military of firing warning shots at its troops near their heavily fortified border, saying it risked raising tensions to "uncontrollable" levels.
South Korea's new leader Lee Jae Myung has sought to ease tensions with the nuclear-armed North and vowed to build "military trust", but North Korea has said it has no interest in improving relations with South Korea.
The latest confrontation occurred on Tuesday as North Korean soldiers worked to permanently seal the frontier dividing the peninsula, North Korea's state media said, citing a statement by Army Lieutenant General Ko Jong Chol.
The incursion prompted "our military to fire warning shots", South Korea's Joint chiefs of staff said in a statement, adding "the North Korean soldiers then moved north" of the de facto border.
Calling the incident a "serious provocation", Mr Ko said South Korea's military used a machine gun to fire more than ten warning shots towards the North's troops, according to the official Korean Central News Agency.
"This is a very serious prelude that would inevitably drive the situation in the southern border area where a huge number of forces are stationed in confrontation with each other to the uncontrollable phase," Mr Ko said.
'Deliberate provocation'
The last border confrontation between the arch-rivals was in early April when South Korea's military fired warning shots after around ten North Korean soldiers briefly crossed the border.
Those troops were spotted in the demilitarised zone between the two countries, parts of which are heavily mined and overgrown.
North Korea's military announced last October it was moving to totally shut off the southern border, saying it had sent a telephone message to US forces to "prevent any misjudgment and accidental conflict".
Shortly after, it blew up sections of the unused but deeply symbolic roads and railroad tracks that connect the North to the South.
Mr Ko warned that North Korea's army would retaliate against any interference with its efforts to permanently seal the border.
"If the act of restraining or obstructing the project unrelated to the military character persists, our army will regard it as deliberate military provocation and take corresponding countermeasures," he said.
'Restore trust'
Under Mr Lee's more hawkish predecessor, relations between the two Koreas had sunk to one of their lowest points in years.
After Mr Lee's election in June, he pledged to pursue dialogue with the nuclear-armed North without preconditions, saying last week his government "will take consistent measures to substantially reduce tensions and restore trust".
Even so, South Korea and the United States began annual joint exercises on Monday aimed at preparing for potential threats from the North.
Mr Lee described the drills as "defensive" and said they were "not intended to heighten tensions".
North Korea - which attacked its neighbour in 1950, triggering the Korean War - has long been infuriated by such exercises between the US and the South, decrying them as rehearsals for invasion.
Hong Min, a senior analyst at the Korea Institute for National Unification, said North Korea was again accusing South Korea of pursuing a "dual approach" with its latest outburst - calling for dialogue while in its view raising military tensions.
North Korea's leader Kim Jong Un called earlier this week for the "rapid expansion" of the North's nuclear weapons capability, citing the ongoing US-South Korean military exercises that he claimed could "ignite a war".
His powerful sister has since said South Korea "cannot be a diplomatic partner" of the North, and that Mr Lee "is not the sort of man who will change the course of history".
Accreditation: AFP
