In a signed commentary, the North's official Korean Central News Agency (KCNA) claimed the joint military drill was aimed at invading the North.
"It is a well-known fact that the US and the south Korean authorities grew proficient in the capabilities for fighting a war, steadily modifying and supplementing operational plans of all descriptions that presuppose a sudden preemptive attack on the DPRK for the past decades," the commentary said.
DPRK stands for the North's official name, the Democratic People's Republic of Korea.
"As acknowledged by the world people, the purpose of the war exercises targeting the DPRK has not changed," it added.
The commentary came one day after the communist state said in another attack against Seoul over the allies' joint military exercise that it will never sit with South Korea for dialogue again.
Pyongyang has recently been thought to be trying to sever ties and dialogue with Seoul in an apparent attempt to enhance its chances of direct dialogue with the US, as well as of securing concessions from Washington.
The KCNA comment, however, blasted the US for violating the countries' denuclearization agreement reached at the historic first summit between US President Donald Trump and North Korean leader Kim Jong-un in Singapore in June 2018.
"This is an open denial and an outright challenge to the historic DPRK-US joint statement in which commitments were made to establish new DPRK-US relations and build lasting and durable peace-keeping mechanism on the Korean Peninsula," it said.
"No law stipulates that the one side may backtrack from commitments and only we have to fulfill them... The US should keep in mind that our repeated warnings are not hot air," it added.
Pyongyang also vowed to continue its weapons development.
"The right answer for removing all the potential and direct threats posed to the security of our state is the constant development of powerful physical means and their deployment for an actual war."
North Korea has been staging a series of missile launches, which it has described as new weapons tests.
The KCNA reported earlier that leader Kim had personally guided the test-firing of a new weapon on Friday morning. (Yonhap)