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.
The start of the two-week annual "Key Resolve" exercise follows a week of escalating tensions on the Korean peninsula, with North Korea lashing out over tightened UN sanctions adopted after its third nuclear test last month.
Pyongyang has condemned the joint manoeuvres as a provocative invasion rehearsal and announced that - effective Monday - it was scrapping the 1953 armistice ending the Korean War with the South.
The South's Unification Ministry confirmed that the North appeared to have carried through on another promise to sever the hotline between Pyongyang and Seoul.
The two sides habitually speak twice a day, but "the North did not answer our call this morning," a spokeswoman for the South's Unification Ministry said.
The hotline was installed in 1971 and the North has severed it on five occasions in the past - most recently in 2010.
'End of ceasefire'
And the Rodong Sinmun, the newspaper of the North's ruling communist party, confirmed in Monday's edition the "complete end" of the ceasefire which halted the 1950-53 Korean War hostilities.
"With the ceasefire agreement blown apart ... no one can predict what will happen in this land from now on," the newspaper said.
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.
Al Jazeera's Scott Heidler, reporting from the South Korean town of Paju, said that while people were concerned over the heightened tensions, others were convinced that the latest developments were merely heightened rhetoric from both sides.
"Key Resolve" is a largely computer simulated exercise, but still involves the mobilisation 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 defence 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.