World Bulletin / News Desk
The weeks-long annual military exercises in the South have also previously provoked Pyongyang, which views them as a preparation for invasion.
This year's drills started amid already heightened tensions following the North's banned ballistic missile test last month and Seoul's insistence that Pyongyang was behind the Feb. 13 chemical poisoning of Kim Jong-nam -- the half-brother of North Korean leader Kim Jong-un.
The reclusive state's Korean People's Army vowed to hit back with its "toughest counteractions" and launch a "merciless" response if just one allied shell is fired into the North's waters.
"This stand of ours clarified before the world is by no means empty talk," the statement insisted via the North's state-run KCNA news agency.
Meanwhile, South Korea's acting leader Hwang Kyo-ahn called for unity at a national prayer meeting Thursday while the country is confronted by a "grave situation at home and abroad”.
In addition to threats from the North, Seoul continues to see mass street protests as the Constitutional Court enters its final deliberations on whether to support or overturn the impeachment of scandal-hit President Park Geun-hye.
According to local news agency Yonhap, Hwang found it "regrettable" that the public is divided -- although he has been the focus of recent rallies for refusing to extend a special investigation into wide-ranging claims of corruption surrounding Park.