Timeline of tensions between the two Koreas

SEOUL - In the 60 years since an armistice was signed ending hostilities in the 1950-53 Korean War, numerous incidents and clashes - most instigated by North Korea - have threatened the official ceasefire.

Because the truce was never ratified by a formal peace treaty, the two Koreas technically remain at war, and Pyongyang has in the past put its troops on a war footing during times of high tension.

Here is a selection of the most significant incidents, some of which came perilously close to plunging the Korean peninsula back into a full-scale conflict.

August 21, 2015 - North Korean leader Kim Jong-Un orders his frontline troops onto a war footing following a rare exchange of artillery shells a day earlier.

November 23, 2010 - North Korea shells the South Korean border island of Yeonpyeong in the Yellow Sea, killing four people - two marines and two civilians.

March 26, 2010 - The South Korean corvette Cheonan sinks near Baengnyong island in the Yellow Sea, killing 46 sailors. An international investigation concludes that it had been torpedoed by a North Korean submarine. Pyongyang denies the charge.

June 29, 2002 - A naval clash at Yeonpyeong results in the sinking of a South Korean patrol boat with the loss of six lives.

June 15, 1999 - South Korean and North Korean naval ships clash off Yeonpyeong. One North Korean torpedo boat is sunk and three patrol boats badly damaged. North Korean casualties were estimated at around 50.

September 1996 - A North Korean submarine on a spying mission runs aground off the eastern South Korean port of Gangneung. Nearly all the 25 crew and infiltrators end up killed after a 45-day manhunt.

November 29, 1987 - A bomb planted by two North Korean agents on a Korean Air flight explodes over the Andaman Sea, killing all 115 people on board. Both agents try to avoid capture by ingesting cyanide but one is taken alive.

October 9, 1983 - North Korea bombs a mausoleum in Yangon, Myanmar, during a visit by South Korean President Chun Doo-hwan. He survives but 21 people, including some government ministers, are killed.

August 18, 1976 - North Korean soldiers attack a work party trying to chop down a tree inside the demilitarised zone (DMZ). Two US army officers are killed in what becomes known as the "axe murder incident".

Jan 21, 1968 - A team of 31 North Korean commandos infiltrates within striking distance of President Park Chung Hee's Blue House office and residence complex, before being intercepted by South Korean security. All but two were killed in a series of ensuing gun battles. One survivor reveals their mission was to assassinate Park.

January 19, 1967 - North Korean artillery units fire on a South Korean vessel, the Dangpo, patrolling in the Yellow Sea with 70 sailors on board. The ship sinks, killing 39 of the crew.