South Korea's military seized a North Korean boat that crossed into its waters on Tuesday and fired a warning shot to see off a North Korean patrol vessel that tried to intervene, Seoul officials said.

South Korea's Yonhap news agency initially described the vessel as a fishing boat, but officials said that had still to be determined.

Seven unarmed sailors were aboard the vessel, but six appeared to be wearing uniforms, a South Korean military official said. During initial questioning, the North Korean sailors claimed that there must have been a "navigating error".

A North Korean patrol boat that was tracking the seized vessel briefly crossed the Northern Limit Line (NLL), the de facto sea boundary, but turned back after the South Korean military broadcast warnings and fired a warning shot, a second military official said.

"We've informed the North side that we're making checks on the boat and will notify the results," the official told reporters.

The boat crossed into the South's waters at around 9:30 a.m. (0030 GMT), and was about 10 km (6.2 miles) off the west coast of the peninsula when it was seized and towed to Baengnyeongdo island for investigation, the official said.

The official said it was the first time that a North Korean patrol vessel had crossed the border since 2018, when the rival Koreas agreed to cease "all hostile acts" and dismantled some structures along the heavily fortified land border.

The incident comes at delicate time on the Korean Peninsula. The South is set to hold a presidential election on Wednesday, and tension has risen over the North's recent weapons tests and fresh signs of activity at its nuclear testing site.

The two Koreas have officially remained in a state of war since the 1950-53 Korean War ended with an armistice not a peace treaty.