Protestors hold placards that read 'stop war exercise' during a rally denouncing the annual Ulchi Freedom Guardian (UFG) joint South Korea-US military exercise, near the US embassy in Seoul, on August 21, 2017
Seoul and Washington began annual war games on Monday, with South Korean President Moon Jae-In warning the nuclear-armed North not to use them as an excuse to perpetuate the "vicious cycle" of tensions.
Tens of thousands of South Korean and US troops are taking part in the "Ulchi Freedom Guardian" joint military drills, a largely computer-simulated exercise that runs for two weeks in the South.
The annual drills are viewed by nuclear-armed Pyongyang as a highly provocative rehearsal for invasion, and it always meets them with threats of strong military counteraction.
Only weeks ago it said it was considering firing a salvo of missiles towards the US Pacific territory of Guam.
Moon described the exercises as "purely defensive in nature" and warned Pyongyang it "must not use it as an excuse to launch provocations that will worsen the situation".
"North Korea must understand its repeated provocations are what is forcing South Korea and the US to conduct the joint defensive drills, which in turn, keeps the vicious cycle going," Moon told a Cabinet meeting.
Pyongyang tested two intercontinental ballistic missiles (ICBM) last month that appeared to bring much of the US within range.
That sparked a grim warning by President Donald Trump that Washington could rain "fire and fury" on the North.
Leader Kim Jong-Un last week delayed the Guam strike plan, but warned it could go ahead depending on Washington's next move.
While the allies are pushing ahead with the exercises that date back to 1976, around 17,500 US troops will participate in the drills -- a cutback from last year.
South Korean media reports have said the US was considering scrapping a plan to bring in two aircraft carriers to the peninsula.
But US Defense Secretary James Mattis said Sunday the smaller troop numbers were "by design to achieve the exercise objectives", denying suggestions Washington had cut them back to try to ease tensions with Pyongyang.
"This right now is an exercise to make certain that we're ready to defend South Korea and our allies over there," Mattis told reporters aboard an aircraft flying to Amman, Jordan.
US Pacific Command chief Admiral Harry Harris arrived in the South on Sunday to inspect the exercises and discuss growing North Korean nuclear and missile threats.
On the eve of the UFG drills, North Korea said the US was "pouring gasoline on fire".
In a commentary carried by the official Rodong Sinmun newspaper, the North warned of an "uncontrollable phase of a nuclear war" on the peninsula, which would entangle the US mainland.
"If the United States is lost in a fantasy that war on the peninsula is at somebody else's doorstep far away from them across the Pacific, it is far more mistaken than ever."