Plea for release
"A preliminary investigation revealed the boat, with four South Koreans and three Chinese on board, intruded into our economic zone ... [The] investigation will continue," the agency added.
The South had sent North Korea a message urging it to release the boat and its crew "in accordance with international laws and customs and humanitarian spirit".
The South's coast guard has said the boat was presumed to have been inside an economic zone proclaimed by the North in the Sea of Japan when it was detained.
The seizure was made during a major South Korean naval exercise in the Yellow Sea, for which the North had threatened retaliation.
The seizure of the 41-tonne fishing boat further heightened tensions between the two sides in the aftermath of the deadly sinking of the Cheonan, a South Korean navy shipin March that resulted in the deaths of 46 sailors.
A five-nation team of investigators concluded in May that a North Korean torpedo sank the Cheonan frigate near the Koreas' western maritime border, though Pyongyang has denied the charge.
The attack was South Korea's worst single military loss of life since the 1950-53 Korean War.