A North Korean patrol boat on Monday crossed the de facto western maritime border between the two Koreas following the North's long-range rocket launch a day earlier, a South Korean military official said.

The South's Navy fired five rounds of warning shots against the North's patrol boat, which trespassed across the maritime border in the Yellow Sea, widely known as the Northern Limit Line (NLL), at around 6:55 a.m., according to the official.

The vessel retreated northward about 20 minutes after it intruded into South Korean territory.

On Sunday, the North launched a long-range rocket carrying what it called an Earth observation satellite, inviting strong international condemnation. Seoul and Washington see the launch as a cover for a banned intercontinental ballistic missile test.

The North's patrol boats have frequently intruded into South Korean territorial waters by crossing the NLL. The communist neighbor has long demanded that the line be moved farther south.

"The South Korean military is on high alert, beefing up surveillance near the NLL and monitoring any abnormal activities by North Korean soldiers," the official said.

President Park Geun-hye has called for more vigilance against North Korea's possible provocations in the wake of the North's Jan. 6 nuclear test and its missile launch.

Gen. Lee Sun-jin, chairman of South Korea's Joint Chiefs of Staff, warned Sunday that North Korea is highly likely to make tactical provocations at an unexpected location and timing, calling for combat readiness.

Pyongyang does not acknowledge the NLL, which was drawn unilaterally by the U.S.-led United Nations Command when the 1950-53 Korean War entered into a cease-fire. The two Koreas fought bloody battles in 1999, 2002 and 2009 near the border. (Yonhap)