The United States said Wednesday its imminent naval exercises with South Korea are "defensive" in nature and meant as a clear show of "deterrence" in the face of North Korean "aggression."

The comments bolstered a major show of American support for Seoul, including the naval drills, new US sanctions on Pyongyang and a trip to the demilitarized zone by administration heavyweights Hillary Clinton and Robert Gates.

China has however expressed fears that the naval maneuvers starting on Sunday could further raise tensions with the isolated North, already dangerously high after the sinking of a South Korean ship, blamed on Pyongyang.

"It's important to understand these are exercises that are defensive in nature," White House spokesman Robert Gibbs said.

"Defense sends a clear signal of deterrence to the aggression of North Korea, and in support of the defense of South Korea."

"Certainly we are strongly supportive of exercises that demonstrate South Korea's defending itself," Gibbs said.

President Barack Obama's chief spokesman however sidestepped a question over China's concerns about the naval drills -- involving 200 planes and 20 ships, including a massive aircraft carrier -- due to start on July 25.

Qin Gang, a foreign ministry spokesman in Beijing, earlier called on all sides in the region to "maintain a cool head and exercise restraint, and not do anything that aggravates regional tensions."

US Defense Secretary Gates and his South Korean counterpart Kim Tae-Young announced the drills on Tuesday, saying they were designed "to send a clear message to North Korea that its aggressive behavior must stop."

South Korea, the United States and other nations -- citing the findings of a multinational investigation -- have accused the North of sending a submarine to torpedo the Cheonan warship near the tense Yellow Sea border in March.

China, North Korea's closest ally and most important source of economic and other aid, has refused to criticize its neighbor over the sinking and repeatedly warned Washington and Seoul against the exercises.

"We resolutely oppose foreign military ships and planes coming to the Yellow Sea and other waters near China to engage in activities that affect China's security interests," Qin said in the statement.

China's concerns were likely to be aired in a meeting between Secretary of State Clinton and Chinese Foreign Minister Yang Jiechi on Thursday in Vietnam.

"We will be consulting with China (on) what we think (are) additional steps that it can take," State Department spokesman Philip Crowley said.

"The secretary will have a chance to explain to Foreign Minister Yang what we have in mind here and the way forward not only on issues related to North Korea but also issues related to Iran," Crowley said.

The North denies involvement in the sinking of the Cheonan, which claimed 46 lives, and says any retaliation could spark war.

The US-led United Nations Command said the July 25-28 drill would involve about 20 ships including the 97,000-tonne aircraft carrier USS George Washington and some 200 fixed-wing aircraft.

Earlier, Clinton said new US sanctions on Pyongyang were intended to pile on more pressure and prevent the regime bankrolling its atomic program or spreading nuclear arms.

The measures also are designed to block money laundering and other illicit activities.