"During the period of military exercises, we cannot guarantee the safety of South Korean civil aircraft crossing over or near our airspace, especially over the Sea of Japan [East Sea]," a North Korean statement said on Thursday.
The statement, released by North Korea's Committee for the Peaceful Reunification of the Fatherland, also said the Korean Peninsula was on the brink of war because of joint military training operations by Seoul and Washington.
The South Korean military said earlier that about 26,000 U.S. troops and an aircraft carrier would participate in the drill, code-named Key Resolve and Foal Eagle. There were no reports of how many South Korean troops would participate.
The U.S. and South Korea have said the 12-day drills are for defensive purposes.
Tensions are high in the region over reports that the North is planning what it calls a launch a telecommunications satellite, but which the U.S. and South Korea suspect to be long-range Taepodong-2 missile from the newly constructed Musudan-ri launch pad on the country's northeast coast.
Relations between Seoul and the communist North have deteriorated since South Korean President Lee Myung-bak took office in February 2008.
After coming to power last February, Lee said he would review agreements reached at the 2000 and 2007 inter-Korean summits, and demand more in return from the North for the economic support provided by Seoul.
North Korea said recently it would scrap all political and military agreements with South Korea, including a non-aggression pact, over its neighbor's "hostile intent." The two countries are still technically at war as their 1950-53 conflict ended in a truce rather than a peace treaty.
