SEOUL, South Korea — A North Korean army sergeant reportedly defected to South Korea on Saturday after killing his platoon and squadron leaders.

South Korean border guards, after hearing gunfire to the north, saw a North Korean soldier crossing the heavily armed land frontier, said a spokesman for the South Korean military’s Office of Joint Chiefs of Staff, speaking on the condition of anonymity because an official announcement had not been made.

The soldier threw down his rifle, after which the guards used a loudspeaker to confirm his intention to defect, according to South Korean military officials. The soldier, identified only as a sergeant, crossed the border shortly after noon. He later told investigators that he had defected after killing two officers, according to the South Korean military, which withheld more details pending further investigation.

Thousands of North Koreans defect to South Korea every year, leaving behind hunger and oppression in their homeland, but almost all of them travel through China. It is unusual for a North Korean soldier to defect through the tightly sealed land border, which is defended on both sides with minefields, barbed-wire fences and armed soldiers.

The last such defection was in March 2010, when a North Korean sergeant crossed the eastern land border. The defection Saturday took place in the west, near a crossing point where South Korean managers enter the North every day to work at an industrial park in the North Korean town of Kaesong, just north of the border. Products made at the complex, a joint venture between the Koreas, enter the South through the same cross-border road.

The two Koreas technically are still at war, the 1950-53 Korean War having ended with a truce, not a peace treaty. Their relations have worsened in the past five years, after President Lee Myung-bak of South Korea refused to send substantial amounts of aid until North Korea ended its nuclear weapons program. In 2010 the North shelled a South Korean border island, killing four people; it is also accused of having sunk a South Korean naval vessel during the same year, an episode in which 46 sailors died.