A large 4-day exercise involving U.S and South Korea forces is under way in the Sea of Japan.
It comes in response to the sinking of a South Korean warship, the Cheonan, four months ago. An international investigation blamed a North Korean torpedo attack for the incident.
The war games off the east coast of the Korean peninsula were called to send a strong message of deterrence to Pyongyang.
The joint military exercise is known as "Invincible Spirit." And while the forces of the United States and South Korea routinely conduct drills together, this war game is considered unprecedented in scope.
For the first time, the most advanced fighters of the U.S. Air Force are participating. They are the F-22 Raptors. U.S. officials say four of the stealth technology jets are taking part, along with nearly 200 other aircraft.
At sea is the strike group of the USS George Washington. The nuclear-powered aircraft carrier is accompanied by three destroyers.
U.S. and South Korean top military officials say the exercises are a significant show of force intended to deter North Korea from further aggression.
South Korea originally said the U.S. aircraft carrier and other vessels would deploy in the Yellow Sea, where the Cheonan incident occurred. That announcement prompted strong protest from China.
South Korean Navy Rear Admiral Kim Kyung-shik denies pressure from Beijing was the main reason the war game was shifted from the western waters.
The admiral says planners considered the objectives for the exercise and what needed to be emphasized for training, as well as the required air and maritime space. That is why, he says, a decision was made to conduct it in the eastern waters, instead of the Yellow Sea.
The shift of location may have mollified China to some extent, but North Korea is reacting with rhetoric typically heard whenever the United States and South Korea conduct joint exercises here.
A North Korean TV newscast announcer Saturday evening quoted a statement for the National Defense Commission: the army and the people will counter the "nuclear war exercise" with atomic weaponry.
North Korea says the maneuvers could compel it "to start a retaliatory sacred war."
Analysts dismiss the statements as bluster and there is no indication of unusual activity among North Korea's forces.
The current escalating tension comes 60 years after North Korean forces crossed into the South, starting three years of combat that ended in stalemate. No peace treaty has ever been signed.