Pyongyang's state media on Sunday called this year's drills a "reckless" move that could trigger the "uncontrollable phase of a nuclear war".
"The U.S. should take the whole responsibility for catastrophic results from its reckless war game as it choose to make a military confrontation with us", its spokesman said in a statement.
Tens of thousands of South Korean and USA 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.
It's an annual joint drill between American and South Korean forces, but this year, it comes following a bitter back-and-forth between North Korea and U.S. President Donald Trump.
The exercise involves 17,500 American troops and 50,000 South Korean soldiers, according to the United States military command in South Korea and Seoul's defence ministry.
"If North Korea provokes again, it will face with much harsher sanction and won't stand it in the end".
South Korea's President Moon Jae-in said Monday that North Korea must not use the drills as a pretext to launch fresh provocation, saying the training is held regularly because of repeated provocations by North Korea. So far the US and South Korea have rejected this idea. "The U.S. has to be responsible for the ensuing disasters". "I want to warn North Korea to do no more unsafe gambling", Moon said.
China's willingness to get tough with North Korea changes the dynamics of the standoff, and makes future negotiations more consequential than past attempts.
North Korea's missile program is banned by United Nations resolutions. During last year's Ulchi drills, North Korea test-fired a submarine-launched ballistic missile that flew about 310 miles in the longest flight by that type of weapon.
"The joint exercise is the most explicit expression of hostility against us, and no one can guarantee that the exercise won't evolve into actual fighting", the North's official Rodong Sinmun newspaper said.
"China should also make clear that if North Korea launches missiles that threaten USA soil first and the United States retaliates, China will stay neutral", it added.
This year, the USA has scaled back the number of troops participating by about eight thousand, but Defense Secretary James Mattis says that's not because of heightened tensions.
Around 17,500 U.S. troops will participate in this year's drills - a cutback from last year - according to numbers provided by Seoul's defence ministry.