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 (JCS) said.