Two North Korean patrol boats crossed the western maritime border and retreated early Friday, the South Korean military said, just hours before U.S. President Barack Obama is due to arrive in South Korea for an official visit.

The fishery inspection vessels came 1 nautical mile south of the maritime boundary, called the Northern Limit Line, in the Yellow Sea and retreated after the South Korean Navy broadcast several warnings and fired warning shots between 4:35 a.m. and 5:03 a.m., the Joint Chiefs of Staff said.

"The military is looking into North Korea's intention, while maintaining high vigilance and putting North Korea under closer watch," a senior JCS official said. 

The North Korean vessels may have been controlling its fishing boats as dozens of Chinese boats were operating in nearby waters at that time as the crab fishing season was entering its peak in the western sea, the official said.

There are also possibilities that the North may have intended to check the South Korean force's military vigilance or draw international attention ahead of Obama's two-day visit to South Korea, a defense ministry official said, asking for anonymity.

Seoul and Washington have stepped up surveillance against possible North Korean provocations ahead of Obama's two-day visit, whose arrival date coincides with the founding anniversary of North Korea's armed forces.

While North Korea has often used big parades of weapons and missile launches to mark one of its biggest national holidays, its military has not staged any special events this year, the spokesman for the South Korean defense ministry said.

Last year, Pyongyang celebrated its military's founding anniversary with a march past of troops attended by leader Kim Jong-un.

Tensions remain high on the Korean Peninsula as increased activities have been detected at North Korea's main underground nuclear test site in its northeastern tip after the communist state threatened to conduct a "new form" of nuclear test late last month.

New satellite imagery at Punggye-ri site has recently shown additional activities, indicating that Pyongyang is close to conducting its fourth atomic test, according to U.S. think tank 38 North.

The website, run by the U.S.-Korea institute at Johns Hopkins School of Advanced International Studies, said Friday the satellite images, taken just two days ago, showed additional activity at the Punggye-ri test site that is "probably related to preparations for a detonation." (Yonhap)