The missiles, launched between 5:45 a.m. and 6:40 a.m. local time, were fired from North Hwanghae province in the western part of North Korea and traveled between 500 and 600 kilometers (between 310 and 370 miles), according to a statement from the South Korean Joint Chiefs of Staff.
According to North Korean state media, Kim Jong Un personally "provided field guidance" for the drill.
The missiles are believed to be short-range, Scud or Rodong type and flew around 500 to 600 kilometers (300 - 380 miles), said Jeon Ha-gyu, spokesman for the South Korean Joint Chiefs of Staff.
The statement noted that this range encompassed all areas of South Korea, including the seaport of Busan at South Korea's southern tip.
The missile test comes six days after South Korea announced it will deploy the advanced U.S THAAD missile defense system
at Seongju County, about 250 kilometers (155 miles) southeast of Seoul.
When active, the system should be able to defend two-thirds of South Korea from an attack by their northern neighbor.
The North Korean drill was conducted "under the simulated conditions of making preemptive strikes" on the regions of South Korea where THAAD is located, state-run KCNA said.
It isn't North Korea's first missile test this month -- on July 9, a ballistic missile was fired
from a submarine off the country's east coast, which shortly afterward fell into the water.
The submarine launch came shortly after the original announcement of South Korea's THAAD defence system and the U.S. placing even stricter sanctions on North Korean leader Kim Jong Un for alleged human rights abuses
Seven months of sabre-rattling
The region has been on edge since North Korea announced it had successfully tested a hydrogen bomb in January
, after a magnitude 5.1 seismic event was detected inside the rogue state.
The U.N. Security Council voted to impose significant punitive measures on the state at the time.
It didn't appear to deter North Korea however -- in June, the country fired two Musudan intermediate-range missiles from their eastern coast, one of which they announced had been a successful test
U.S. and South Korean officials said out of the two tests, one had flown just 150 kilometers (93 miles) before plunging into the Sea of Japan, while the other had traveled 400 kilometers (249 miles).
In addition to the June tests, North Korea had already fired four other Musudan missiles in 2016, including one on the anniversary of Kim Il Sung's birthday in April -- all were reporte to have failed.