South Korea's military said on Wednesday it had sent back all seven North Korean sailors who were aboard a ship it seized after the vessel crossed into the South's waters a day before.

The boat was towed to a South Korean border island after crossing the Northern Limit Line (NLL), the de facto sea boundary between the two Koreas, according to Seoul's military officials.

A North Korean patrol vessel that was tracing the seized ship briefly crossed the NLL but turned back shortly after the South's military broadcast warnings and fired a warning shot.

Seoul's defence ministry said its military handed the seven sailors and the seized boat to North Korean authorities near the NLL around 14:00 p.m. (0500 GMT), after the crew asked to return during questioning.

"It's been confirmed that the border crossing resulted from a navigating error and mechanical defects, and all of the sailors expressed their willingness to go back to North Korea," the ministry said in a statement.

The incident came at a sensitive time on the Korean Peninsula. The South holds a presidential election on Wednesday, and tensions have risen over Pyongyang's recent missile tests and rising signs of activity at its nuclear testing site.

The two Koreas remain technically at war since the 1950-53 Korean War ended with an armistice, not a peace treaty.