SEOUL, NOV. 21 -- North Korean border guards fired rifle shots today at a South Korean patrol along the demilitarized zone between the two countries, injuring a South Korean soldier, the Defense Ministry charged.

The ministry refused to disclose details of the incident, but said in a statement that the North Koreans made an unprovoked attack, forcing the South Korean patrol to fire back in self-defense. The ministry did not disclose the number of soldiers involved in the firefight, or the number of rifle shots fired in the tense DMZ.

One South Korean soldier was injured when a bullet struck his left shoulder, the ministry said. The soldier was taken by helicopter to a nearby hospital, where his condition was described as not critical.

The incident marks the first shooting conflict in the DMZ since the South Korean election campaign got under way, and comes at a time of increased tension between Seoul and Pyongyang over the 1988 Summer Olympics, to be held in Seoul. The United States, which has 40,000 troops in South Korea, has warned Pyongyang against any provocative acts aimed at sabotaging the election or the Olympics.

Seoul and Washington fear that North Korea, stymied in its attempt to cohost the Olympics, may try to raise border tensions in a bid to scare some security-conscious countries from participating in the games.

"We believe you are trying to set the stage for serious incidents that you hope will discourage the nations of the world from participating in the 1988 Olympics," U.S. Rear Adm. William Pendley warned North Korean Maj. Gen. Lee Tae Ho at a September meeting of the Military Armistice Committee, which oversees the fragile truce between Pyongyang and Seoul.

Today's shooting incident is reportedly the second this year, and part of a decades-long history of border clashes between soldiers of the two sides and their allies. The U.S.-led United Nations Command filed a protest over the incident, which occurred at about 2:20 p.m. local time near Chorwon, northeast of Seoul, the Defense Ministry said.