Military officials said that despite threats of retaliation, North Korea was staying clear. Most of the firepower for the four-day exercises — which North Korea condemns — has been flying off the decks of the USS George Washington, a U.S. supercarrier that can carry up to 70 aircraft and more than 5,000 sailors and aviators.
Washington and Seoul are hoping the drills — and the deployment of the most potent symbol of American military reach in the U.S. Navy — will send a powerful message to North Korea in the wake of the March sinking of a South Korean warship that killed 46 sailors. An international investigation determined the ship was sunk by torpedo, likely in a sneak attack by a North Korean submarine.
"The message is in the eye of the beholder," said Rear Adm. Daniel Cloyd. "But we would hope that they would take this and messages in the future as a message of resolve. We hope this will give them pause."
North Korea — which has denied any connection to the sinking — has threatened to counter the maneuvers with some sort of military show of its own. But on the second day of the maneuvers, scheduled to run through Wednesday, officials said no signs have been seen the North will make good on its saber-rattling rhetoric.
Cmdr. Ray Hesser, head of an anti-submarine helicopter squadron on the George Washington, said North Korean submarines are largely restricted to shallow, coastal waters.
"We're not expecting to see them out here," he said. "I would not think they would be willing or wanting to come all the way out here."
He said the South Korean warship, the Cheonan, may have been unprepared when the attack occurred, noting that U.S. ships observe higher readiness. It was the worst military attack on South Korea since the 1950-53 Korean War.
"It was like a sucker punch," Hesser said. "It doesn't say much about how much of a fighter you are."
The "Invincible Spirit" exercises involve about 20 ships, 200 aircraft and about 8,000 U.S. and South Korean sailors. It is the Japan-based George Washington's first deployment to South Korea since 2008.
The anti-submarine phase of the training — which also involves anti-ship and anti-aircraft operations — is particularly important because it helps ready the U.S. and South Korean navies to respond to scenarios focused on North Korean submarine activities.
"I am concerned about every submarine underwater that I don't know about," Capt. David Lausman, the carrier's commanding officer, said Monday.
Lausman said the attack demonstrated the opaque nature of Pyongyang's military, which he said should not be underestimated.
"North Korea's danger lies because they are unpredictable," he said. "The sinking of the Cheonan is a prime example."
North Korea has strongly protested the exercises, saying they are a provocation, and has threatened retaliation. In flourishes of rhetoric typical of the regime, it vowed to respond with a "sacred war" and a "powerful nuclear deterrence."
"Should the U.S. imperialists and (South Korea) finally ignite a new war of aggression ... (North Korea) will mobilize the tremendous military potential including its nuclear deterrence for self-defense and thus wipe out the aggressors," North Korea's defense chief, Kim Yong Chun, said in Pyongyang on Monday, according to the official Korean Central News Agency.
North Korea often cites the U.S. military presence in the region, with nuclear-powered warships like the George Washington, as a key reason it needs atomic weapons.
U.S. officials say that the maneuvers, held well away from North Korea's border, are not intended to provoke a response, but are a message that further aggression will not be tolerated.
On Monday, Gen. Han Min-goo, chief of the South Korean Joint Chiefs of Staff, toured the George Washington to demonstrate the allies' solidarity.
The exercises are the first in a series of U.S.-South Korean maneuvers conducted in the East Sea and in the Yellow Sea closer to China's shores. They are the first to employ F-22 jets — stealth fighters capable of evading North Korean air defenses — in South Korea.
The North routinely threatens retaliation when South Korea and the U.S. hold joint military drills, which Pyongyang sees as a rehearsal for an invasion. The U.S. keeps 28,500 troops in South Korea and another 50,000 in Japan, but says it has no intention of invading the North.
However, the sinking of the Cheonan has heightened tensions in the region. North Korea says the investigation results were fraudulent and has warned against any punishment.
Still, Capt. Ross Myers, commander of the George Washington's air wing, said the threats were being taken seriously.
"There is a lot they can do," he said. "They have ships, they have subs, they have airplanes. They are a credible threat."