North Korea regularly protests the annual exercises, which it calls a rehearsal for war, and has recently stepped up its own air, sea and ground military exercises, amid a period of increased tension between the rival Koreas.
"The DPRK will wage a merciless sacred war against the US now that the latter has chosen confrontation," the country's official KCNA news agency said, quoting from an article in the ruling Workers' Party newspaper, the Rodong Sinmum.
DPRK is short for Democratic People's Republic of Korea, the country's official name.
"Nuclear weapons are not a monopoly of the US," the article said. "The US is seriously mistaken if it thinks its mainland is safe."
North Korea frequently makes such threats against the United States and South Korea, which said on Tuesday the two would begin eight weeks of joint military drills from March 2, Reuters reported.
On Friday it said the United States was "much upset by the fact that there may be a sign of detente on the Korean peninsula, thanks to the DPRK's initiative and efforts to achieve peace this year".
However, overtures for dialogue by both Koreas in recent months have stalled, with Pyongyang recently describing inter-Korean relations as "inching close to a catastrophe."