A North Korean patrol ship violated the tensely guarded western maritime border several times Monday night, but it retreated after repeated South Korean military warnings, Seoul's defense ministry said Tuesday.

The North Korean vessel crossed the Northern Limit Line, a de facto maritime border, at around 10:46 p.m. Monday, and sailed to a location about 23.4 kilometers west of South Korea's Yeonpyeong Island in the Yellow Sea.

The ship returned to its territory at around 2:25 a.m. Tuesday after the South Korean military broadcast warnings 10 times, the defense ministry's spokesman said.

"The North Korean ship's NLL violation is seen as part of military drills or an inspection of (the South Korean military)," Kim Min-seok said in a briefing. "It is believed that (the North Korean vessel) intended to test the South Korean military."

The border crossing took place while reunions of families separated by the 1950-53 Korean War were under way in North Korea's Mount Kumgang resort on the east coast.

It also came as South Korea and the United States began their annual joint military drills despite the North's repeated calls to cancel them.

North Korean ships violated the tensely guarded western border three times last year, while this week's border crossing was the first this year.

"We are closely looking into possibilities that the North Korean intentionally violated the NLL," Kim said, stressing the South Korean military is prepared to counter any provocations.

Seoul's prompt announcement of the maritime border violation was seen as an effort to highlight its firm military readiness despite a reconciliatory mood with Pyongyang in light of the family reunions held in three years.

"The military decided to make public (the border trespass) because people have a lot of interest in the North Korean military's moves," Kim said, noting there have been no other unusual moves by the North Korean military.

The much-anticipated Key Resolve and Foal Eagle exercises began on Monday despite Pyongyang's repeated calls earlier this month for their cancellation by threatening to scrap the reunions.

Seoul and Washington are expected to stage a relatively low-profile drill this year so as not to stimulate the North, but their drills drew fierce criticism from the communist state.

On Tuesday, the North's state-run Rodong Sinmun newspaper countered claims by South Korea and the U.S. that the drills are defensive in nature, calling the Key Resolve as a "war rehearsal" that raises tension on the Korean Peninsula.

"(The drills) are a vicious challenge against our efforts to improve inter-Korean ties and defuse tension on the Korean Peninsula," the paper said in a commentary. "We have done all we can to promote peace and stability on the peninsula ... If the warmongers misjudge our willingness for peace, we will sternly respond for the sake of safety and dignity of our people." 

The NLL is considered by South Korea as a maritime boundary with North Korea, while Pyongyang has rejected the legitimacy of the line, drawn unilaterally by the U.S.-led United Nations Command when the 1950-53 Korean War ended. Pyongyang has long demanded that the border be drawn farther south.

Waters near the NLL have been the scene of bloody skirmishes in recent years. North Korean fishing vessels often wander into the area and are frequently chased away by South Korean patrol vessels. (Yonhap News)