South Korean and US troops have launched a joint military exercise as North Korea, which has slammed the drill and threatened both countries with nuclear attack, severed its hotline with Seoul.
As the war concluded with a military armistice rather than a peace treaty, the two Koreas remain technically at war.
Sabre-rattling and displays of brinkmanship are nothing new in the region, but there are concerns that the current situation is so volatile that just one accidental step could escalate into serious confrontation and conflict.
"Key Resolve" is a largely computer simulated exercise, but still involves the mobilization of more than 10,000 South Korean and 3,500 US military personnel. About 28,500 US troops are stationed in South Korea.
"This year is particularly important, because it is the first time the (South Korean) Joint Chiefs of Staff have planned and executed this combined exercise," said US General James Thurman, head of the Combined Forces Command.
South Korea is scheduled to assume wartime operational control of the combined forces in December 2015.
The South Korean defense ministry says North Korea is expected to carry out its own large-scale military drill along its eastern front this week, involving the army, navy and air force.
North Korean artillery bases on western islands close to the disputed maritime border have already stepped up drills and placed their cannon in firing positions, ministry officials said.
Last week, North Korea leader Kim Jong-Un told his troops to prepare for "all-out war" as he toured the units responsible for launching an artillery attack on a South Korean island in 2010 that killed four people.
PHOTO CAPTION
US war ships participate in the joint military exercises, dubbed Key Resolve, between South Korea and the US [AP]
Aljazeera