SEOUL, South Korea: Efforts to establish a formal peace in Korea suffered a setback after a naval skirmish between North and South Korean gunboats Thursday. The North Korean vessel, which crossed into South Korean waters while escorting fishing vessels, fired three shots at the southern boat, which was on the scene to investigate the intrusion. The South Korean ship responded with two shots of its own, the South Korea's Defense Ministry said. No injuries were reported. But the incident was serious enough for South Korea to convene an emergency meeting of security-related Cabinet ministers. Although South Korean officials later said the intrusion appeared unintentional, they said a formal protest would be filed with the U.N. Armistice Commission , the organization which monitors the fragile truce that ended the 1950-53 Korean War.