A U.S. Air Force U-2 Dragon Lady takes part in a drill at Osan Air Base in Pyeongtaek, South Korea, August 21, 2017.
North Korea has previously issued similar warlike rhetoric whenever US and South Korean troops conducted major joint exercises.
South Korea's president, Moon Jae-in, said on Monday the drills are defensive in nature.
"North Korea must understand that it is because of its repeated provocations that South Korea and the USA have to conduct defensive exercises, which in turn keeps the vicious cycle going", Moon was quoted as saying at a Cabinet meeting on Monday by Yonhap News Agency.
President Donald Trump has warned that North Korea will face "fire and fury" if it threatens the United States.
On the same day, Pyongyang also stepped back from its plan to strike Guam with four medium-range ballistic missiles. During last year's Ulchi drills, North Korea test-fired a submarine-launched ballistic missile that flew about 500 kilometers (310 miles) in the longest flight by that type of weapon.
North Korean leader Kim Jong-un visited a military unit near the tense inter-Korean border earlier this month when he was absent from public appearances for 15 days, government sources said Tuesday.
South Korean media reports have said the US was considering scrapping a plan to bring in two aircraft carriers to the peninsula. At that time, he delayed a plan to fire four missiles toward the US territory of Guam.
This year, the US 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.
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.
Mattis told reporters while travelling to Jordan that the exercise had been planned months in advance and the focus this year was on integration operations.
The top USA military commander in the Asia-Pacific region arrived in Seoul on Sunday to observe the exercise. Its top propaganda paper, the Rodong Sinmun, on Sunday wrote the exercise is "the most explicit expression of hostility against" North Korea, would be like "pouring gasoline on fire" and worsen the state of the peninsula.
US Pacific Command chief Admiral Harry Harris arrived in the South yesterday to inspect the exercises and discuss growing North Korean nuclear and missile threats.
The North's military says it will launch an unspecified "merciless retaliation and unsparing punishment" on the United States over the Ulchi Freedom Guardian drills that began Monday.
The statement said Turnbull had made "reckless remarks that the allies including Australia were together with the USA and that ANZUS stood for the mutual defense between the U.S and Australia, should either one of them come under attack, and Australia would back the U.S.in time of emergency".