TOKYO, Japan — The United States and South Korea have launched a large-scale military exercise in the Sea of Japan in a demonstration of firepower designed to tame North Korean saber-rattling four months after the regime torpedoed a South Korean navy ship.

Operation Invincible Spirit went ahead despite threats from Pyongyang that it would use nuclear weapons to launch a "retaliatory sacred war" against the U.S. and its ally.

As U.S. officials refused to be drawn into war of words with the regime, the USS George Washington, a nuclear-powered aircraft carrier, led air and naval maneuvers in waters South Korea refers to as the East Sea.

Over the next four days 8,000 American and South Korean troops, 200 fixed-wing aircraft and 20 ships will test the countries' combat readiness against a backdrop of rising tensions and a flurry of belligerent statements from Pyongyang.

While joint military exercises are common, and routinely prompt outbursts from North Korea, this week's maneuvers have taken on added significance in the wake of the March 26 sinking of the Cheonan, the deadliest attack on the South since the 1950-53 Korean War.

U.S. military officials said the exercises were designed to pressure the North into modifying its increasingly erratic behavior.

"These defensive, combined training exercises are designed to send a clear message to North Korea that its aggressive behavior must stop, and that [South Korea] and the U.S. are committed to enhancing our combined defensive capabilities," General Walter Sharp, commander of U.S. forces in Korea, said in a statement.

China, North Korea's only ally and biggest aid donor, had condemned the military exercises as "provocative" at a time of heightened tensions.

In an apparent attempt to placate Beijing, the USS George Washington and other vessels will not operate, as initially planned, near Chinese waters in the Yellow Sea — a flashpoint for North-South naval clashes and scene of the Cheonan sinking, in which 46 sailors died.

North Korea denies attacking the Cheonan and has threatened all-out war if it is unfairly punished for the tragedy. The U.N. Security Council condemned the sinking, but stopped short of blaming Pyongyang.

Read more about the situation inside North Korea.

North Korea's national defense commission, led by the country's president, Kim Jong-il, said the regime would launch a "retaliatory scared war" against the U.S. if the exercises went ahead.

"The army and people of the [North] will legitimately counter with their powerful nuclear deterrence the largest-ever nuclear war exercises," the commission said in a statement carried by the country's official Korean Central News Agency.

But amid the bombast, there are signs that Pyongyang may be attempting to divert attention from the Cheonan sinking. In recent days it has hinted that it is ready to return to nuclear talks for the first time since March 2008, possibly in an attempt to win desperately needed aid in return for dismantling its nuclear program.

The North Korean economy has been severely weakened by international sanctions imposed in the wake of nuclear tests in 2006 and in 2009 and a currency revaluation late last year that wiped out private savings and sent the price of food skyrocketing.

And a new wave of U.S. sanctions, announced last week, are aimed at punishing North Korea's elite and stamping out illegal activities used to raise money for it its nuclear weapons program.

South Korea media said the measures would target 200 North Korean-held foreign bank accounts thought to be connected with activities such as nuclear weapons development, drug trafficking and counterfeiting.

Today's drills come days after the U.S. secretary of state, Hillary Clinton, visited South Korea to demonstrate Washington's steadfast support for its ally, which hosts 28,500 American troops.

Accompanied by the U.S. defense secretary, Robert Gates, Clinton urged North Korea to end its "campaign of provocative, dangerous behavior."

"It is distressing when North Korea continues its threats and causes so much anxiety among its neighbors and the larger region," she told reporters after leaving Seoul to attend an Asian security forum in Hanoi.

"But we will demonstrate once again with our military exercises ... that the United States stands in firm support of the defense of South Korea and we will continue to do so."

South Korean media said the drills and Clinton's visit, which included a highly symbolic visit to the heavily fortified border separating the two Koreas, had taken on added significance in light of China's apparent acceptance of Pyongyang's version of events surrounding the Cheonan tragedy.

"South Korea and the U.S. have no choice but to hold these talks and launch joint military exercises to prevent further provocations while sending a clear message to the North and China that peace and stability must be maintained on the Korean peninsula at all costs," the Chosun Ilbo said.

But the newspaper warned that the region could be entering a period of dangerous instability.

"If China seeks to bolster its alliance with North Korea after witnessing the show of support between South Korea and the U.S., the peninsula could once again become the scene of a Cold War. Seoul and Washington must find a solution to that problem."