Annual military exercises have got underway involving tens of thousands of troops, amid a new threat from North Korea. Pyongyang has threatened a pre-emptive nuclear strike.
The "Ulchi Freedom Guardian" drills that began Monday will last for 12 days and are largely computer-simulated war games.
Around 50,000 South Korean and 30,000 US soldiers are taking part in the war games that play out a full-scale invasion scenario by nuclear-armed North Korea. Both Seoul and Washington insist the drills are purely defensive in nature.
The joint exercises follow what the "Korea Times" newspaper described as the largest-ever South Korean drill on Saturday close to North Korean border, exactly a year after the North fired several artillery rounds towards a South Korean frontline military unit.
The annual drill always triggers an aggressive response from Pyongyang, this year's, however, has come just days after a senior North Korean diplomat defected to the South. The North denounced the defector, Thae Yong Ho, as "human scum."
Pyongyang has also reacted angrily to a US plan to place a high-tech defense missile system in South Korea.
On Monday, the Korean People's Army (KPA) issued a statement, accusing South Korea and the United States of carrying out an invasion rehearsal and warned of a tough military response.
The North's military said it would turn Seoul and Washington into "a heap of ashes through a Korean-style pre-emptive nuclear strike" if they show any signs of aggression toward the North's territory.
Pyongyang's frontline units were "fully ready to mount a preemptive retaliatory strike at all enemy attack groups involved," said a spokesman for the KPA general staff.
Direct dialogue unlikely
Tensions remain high following the North's fourth nuclear test in January and other recent missile launches. In response to substantially increased international sanctions, Pyongyang has shut down the two existing hotlines with South Korea and its only direct communications link with the United States.
After last week's defection of a key diplomat in the North's London embassy, South Korea's Unification Ministry has warned of possible terror attacks or assassination attempts on defectors and anti-Pyongyang activists in the South.
"With the start of the UFG drills tomorrow (Monday), we ask all citizens to be on guard for North Korean terror threats and cooperate with the government's efforts," said the unification ministry official.
South Korean President Park Geun-hye was cited by the "Korea Herald" newspaper Monday, describing a higher likelihood of unrest in Pyongyang, warning of "serious cracks" in the communist regime that could spill over to the South.
mm/sms (AFP, AP)