Just last week, Pyongyang said it had finalized a plan to fire four missiles toward the U.S. territory of Guam.
Pyongyang has now threatened to take action if the USA and South Korea proceed with the joint military exercises.
The situation on the Korean Peninsula remains tense.
Tensions around Pyongyang's missile program have flared up in the recent weeks, following the adoption of the UN Security Council sanctions, which led to North Korea trading threats and warnings with the United States. The South Korean military considers the visit an unusual act and is preparing to prevent a possible military provocation, Yonhap said.
The Ulchi Freedom Guardian drill is a computer simulated defencive exercise created to enhance readiness, protect the region and maintain stability on the Korean peninsula, the Pentagon said.
The South Korean and U.S. militaries on Monday began their first major annual military exercise amid threats from North Korea.
While there has been a concerted effort from some members of the Trump Administration to downplay the risk of an imminent nuclear war with North Korea, the possibility of such a conflict is still palpable, with White House officials openly talking about launch a "preventative war" to resolve the situation. In 2016, it fired a submarine-launched ballistic missile on the third day. Ministry officials said the size is similar to that of previous year, roughly 50,000.
- Where did North Korea get its missile technology? However, the US and South Korea maintain they are purely defensive.
The drills are largely computer-simulated war games held every summer and have drawn furious responses from North Korea.
Despite the threat, US and South Korean militaries launched the annual training event this morning as scheduled.
But US Defence Secretary James Mattis said yesterday 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.
"If the United States is lost in a fantasy that war on the peninsula is at somebody else's door far away from them across the Pacific, it is far more mistaken than ever".
"This is an exercise for the defense of Korea".
In recent times, however, calls have been growing for the US and South Korea to suspend their military drills.