North Korean threats to turn Seoul into a "sea of flames" in the event of any provocation.The Key Resolve/Foal Eagle drills are the first of their type since the communist state's deadly shelling of a South Korean border island last November.
The U.S. and South Korea said the exercises are defensive in nature while training their forces "to respond to any provocation." North Korea habitually denounces them as a rehearsal for invasion.
"If the aggressors launch provocation for a 'local war' the world will witness unprecedented all-out counteraction on the part of the army and people of the DPRK (North Korea)," Pyongyang's military said Sunday.
"It will also see such merciless counteraction as engulfing Seoul in sea of flames..."
Ruling party newspaper Rodong Sinmun Monday accused Seoul of "despicable sycophancy" towards Washington and said the war games increase the risk of nuclear war on the peninsula.
The North routinely issues such warnings before military drills in the South, which has a defense alliance with the United States dating back to the 1950-53 Korean War. Some 28,500 U.S. troops are based there.
But tensions are high after the island attack, which killed two marines and two civilians. The South also accuses its neighbor of torpedoing a warship near the disputed Yellow Sea border in March 2010 with the loss of 46 lives, a charge it denies.
Military talks aimed at thawing icy relations broke down in February. Seoul's unification ministry said Pyongyang's latest comments were "not at all helpful" in improving ties.
The North's regime is also trying to block news of popular revolts that have swept away despotic regimes in North Africa.
The South's military has been floating balloons into the North carrying news of the uprisings along with basic household goods that are in short supply in the impoverished nation, a politician said last week.
Private activists also frequently launch propaganda leaflets, suspended in bundles under the balloons.
On Sunday, the North's military threatened to open fire at sites from where "rotten videos" and other propaganda material are launched.
The two sides halted their official cross-border propaganda barrage under a 2004 deal, but the South said it would partially revive the campaign in response to the warship sinking.
A unification ministry official confirmed the military has been launching leaflets.
"Our government will not retract what it has to do because of protests by North Korea," he told news hounds. "The government will decide on the level of its action by considering various circumstances."
A senior government official said the North appears to see the leaflets "as a considerable threat to its system."
The drills launched Monday involve 12,300 U.S. troops and some 200,000 South Korean service members including reservists.