Tensions between North and South Korea, which remain technically at war, have sparked military clashes on and off since a 1953 ceasefire was signed.
Here is a timeline of major incidents including some involving U.S. forces allied with the South:
January 1968 - Thirty-one heavily armed North Korean spies infiltrate deep into Seoul with a mission to assassinate then President Park Chung-hee; 28 are shot to death.
January 1968 - North Korea seizes U.S. spy ship USS Pueblo and for 11 months holds captive its crew of 83, one of whom dies.
Apr. 1969 - A North Korean MiG-17 fighter jet shoots down a U.S. spyplane killing all 31 aboard.
August 1976 - North Korean soldiers kill two U.S. military officers using axes in a dispute over trimming a tree in the Demilitarized Zone (DMZ).
October 1983 - Seventeen senior South Korean officials including several cabinet ministers are killed when a bomb goes off in the capital of Burma, now Myanmar. A Burmese court rules that it was a North Korean plot.
November 1987 - A Korean Air flight departing Baghdad and bound for Seoul explodes in mid-air over the Bay of Bengal, killing 115. A North Korean spy is convicted by a South Korean court for planting the bomb. Another North Korean believed to have been involved commits suicide.
September 1996 - A North Korean submarine on a spy mission runs aground in the South and the crew flee, sparking a massive manhunt. More than 30 people die, including 24 of the crew, 11 of whom are found together dead.
December 1998 - South Korean navy sinks a submersible North Korean spy vessel off the east coast of the peninsula. One North Korean scuba diver is found dead.
June 1999 - At least 17 and as many as 80 North Korean sailors are killed in naval clashes near the disputed maritime border between the Koreas, the Northern Limit Line (NLL), in the Yellow Sea. One of the North's vessels is sunk, others damaged. The battle follows nine days of incursions by the North into South Korean waters.
June 2002 - In a bloody skirmish between South and North Korean naval vessels in the Yellow Sea, one South Korean frigate is sunk and six South Korean sailors die. As many as 13 North Koreans may have perished.
July 2003 - South Korea says its troops returned machine gun fire after the North shot at an observation post in the DMZ between the two states.
May 2006 - Two North Korean soldiers enter the DMZ and cross into South Korea. They return after South Korean soldiers fire warning shots.
July 2008 - A 53-year-old South Korean housewife, out for morning stroll, is shot dead by the North Korean military on a restricted beach near North Korea's Mount Kumgang. The South Korean government suspends tours to North Korea
November 2009 - The two Koreas engage in a brief naval fight just south of their maritime border damaging vessels from both sides.
January 2010 - The two Koreas exchange artillery fire near the maritime border.
March 2010 - The South Korean navy corvette Cheonan is torpedoed, killing 46 sailors. Seoul blames the North for the attack, which Pyongyang denies. A joint civilian and military investigation team including experts from the United States, Australia, Britain and Sweden, concluded that a North Korean submarine fired the torpedo.
November 2010 - North Korea fires dozens of artillery shells at the island of Yeonpyeong hitting a South Korean military base and setting houses ablaze. Two soldiers are killed and 20 people are injured. South Korea says it was conducting military exercises before the exchange but insists test firings were not aimed toward North Korea.
