SEOUL — South Korea’s Navy fired on a North Korean fishing vessel that crossed the maritime border on Wednesday morning, the second exchange of fire between the two nations in five days.

A spokesman for the Ministry of National Defense said a South Korean ship fired 10 “attention” shots at the North Korean vessel between 7 and 9 a.m. The spokesman said the fishing boat — which was not hit during the incident — had traveled six-tenths of a mile past the Northern Limit Line. Before firing the shots, the South Korean ship sent three radio messages telling the North Korean vessel to turn around, the spokesman said. He said it was unclear why the ship had crossed into South Korean waters.

U.S. Forces Korea spokesman David Oten referred questions to South Korea officials, saying the incident was a matter of peacetime enforcement of the armistice that ended the Korean War. The U.N. command is gathering information about the incident but is not conducting an official investigation, he said.

Oten also said the U.S. military does not as a matter of policy discuss when forces or bases are placed on heightened alert.

It’s the second incident between the two countries in less than a week. On Friday, North Korea fired two rounds across the Demilitarized Zone toward a South Korean guard post around 5 p.m. South Korea fired three rounds in response, but nobody on either side was injured, the MND spokesman said.

Wednesday’s incident comes the week before the G-20 summit in Seoul. The capital has been on heightened alert since last week, particularly because of concerns that North Korea could try to deflect attention from the event or embarrass its neighbor.

“The South Korean government is looking at (various moves that can be taken by) North Korea and terrorist groups and making thorough preparations,” South Korean President Lee Myung-bak said at a news conference Wednesday in Seoul, according to Reuters.

Security forces in South Korea have been put on high alert, anti-aircraft missiles are at the ready, shipping and air routes are under heightened surveillance, and airport screening increased, Reuters reported.

The MND spokesman said Wednesday’s incident at the Northern Limit Line is believed to be unrelated to the G-20.

Tensions have been higher between the two countries since the March sinking of a South Korean navy ship, in which 46 South Korean sailors died. South Korea has accused the North of torpedoing the ship, which North Korea denies.

Sign Up for Daily Headlines

Sign up to receive a daily email of today's top military news stories from Stars and Stripes and top news outlets from around the world.

Sign Up Now