As if on cue, a North Korean patrol boat on Thursday briefly crossed the Northern Limit Line, the de facto maritime border, but sailed back soon afterward.

The border violation comes a day after the South Korean and U.S. defense ministers called on North Korea to respect the line.

An officer with the Joint Chiefs of Staff said the North Korean patrol boat crossed the NLL at about 11 a.m. and sailed 0.37 km into waters east of Baeknyeong Island, apparently trying to crack down on Chinese fishing boats illegally operating there. When a South Korean Navy speedboat broadcast a warning, it went back across the NLL around 11:07 a.m.

It was the first time since June that a North Korean patrol boat has crossed the NLL. However, North Korean fishing boats violated the NLL several times in September.

South Korean military authorities believe Thursday's crossing was an accident. At the time, about 70 Chinese fishing boats were operating in nearby waters. The North Korean patrol boat was seen seizing a Chinese fishing boat that was operating in waters just south of the NLL.

"It seems that the North Korean patrol boat crossed the NLL to seize the Chinese fishing boat though it was supposed to operate in waters north of the line," a military source said. "We spotted no unusual movements by the North Korean military."

When North Korean fishing boats crossed the line last month, the North also readied artillery batteries and kept patrol boats on standby.

"It seems that North Korean patrol boats are making sure to keep their fishing boats north of the line after our Navy fired warning shots at trawlers that crossed the NLL on Sept. 21," said another source.

But South Korean military authorities are mindful that North Korean trawlers crossed the line nine times last month and this may be a concerted attempt to ratchet up tensions and thus influence the upcoming presidential election in the south.

"We're closely watching the North Korean military's movements because we worry that the North may continue minor provocations ahead of the presidential election," the JCS officer added.