The South Korean defence ministry has reported that a patrol ship from North Korea had made tat least three times incursions into its territorial waters.
Following several warnings from the South Korean forces, the vessel retreated, said the ministry.
"The North Korean ship's NLL [Northern Limit Line] violation is seen as part of military drills or an inspection of [South Korean military]. It is believed that [the North Korean vessel] intended to test the South Korean military," spokesperson Kim Min-seok told reporters.
The maritime border remains a contentious issue between the two Koreas as the boundary was unilaterally created by UN forces subsequent to the 1950-53 Korean War. The region had witnessed skirmishes occasionally.
However, no shots were fired but only warnings were issued by South Korea's naval forces during the latest confrontation.
"We are closely looking into possibilities that the North Korean intentionally violated the NLL. The military decided to make public [the alleged incursion] because people have a lot of interest in the North Korean military's moves," said a South Korean spokesperson.
Although such reported violations by North Korea are not uncommon, the timing of the intrusion has raised tensions.
The incident has occurred on the first day of the US-South Korean war games, which are one of the largest military exercises conducted by the two countries, with nearly 200,000 troops from both countries taking part.