The US and South Korea are conducting annual military drills which consistently infuriate Pyongyang, despite appeals to halt the exercise, BBC reported.
Last week North Korea appeared to back down from a threat to send missiles towards the US Pacific island of Guam, but said it would watch US actions.
It has already condemned these drills as pouring "gasoline on fire".
Washington describes the drills as defensive in nature, but the North sees them as preparation for invasion.
China and Russia had in July proposed a halt on military exercises in exchange for a freeze on missile tests.
But Joseph Dunford, chairman of the US Joint Chiefs of Staff, said last week that the military exercises were "not currently on the table as part of the negotiation at any level" and the Ulchi-Freedom Guardian (UFG) exercises were going ahead as planned.
About 17,500 US troops and 50,000 South Korean troops are involved in the exercises, which will last for about 10 days.
After North Korea's threats on Guam and an almost unprecedented war of words over Pyongyang's repeated missile tests, analysts have warned that the joint drills may be seen as a provocation at a particularly sensitive time.
On Sunday an editorial in North Korea's official government newspaper, the Rodong Sinmun said the exercises would worsen the state of the peninsula and warned of an "uncontrollable phase of a nuclear war".
South Korea's President Moon Jae-in responded on Monday that Pyongyang should not use the exercises "as a pretext for aggravating the situation", reported Yonhap news agency.
The drills have also been met with some opposition in South Korea, where protests were held on Monday.
The US and South Korea hold two sets of war games every year, involving a massive number of troops and military hardware.
Foal Eagle/Key Resolve is usually held in spring, while Ulchi-Freedom Guardian (UFG) is in autumn.
Both involve land, sea and air military drills and computer simulations. Held in South Korea, they have also involved practice drills for terror and chemical attacks in recent years.