Jet fighters roared off the deck of this massive carrier Tuesday as the US and South Korean navies staged their biggest joint exercise yet — a show of strength against North Korea.

The 97,000-ton USS George Washington, with a 4.5-acre (1.8-hectare) flight deck, is the centrepiece of a drill which began Sunday, five days after the North's deadly shelling of a South Korean border island.

The hulking grey carrier ploughed through the Yellow Sea at about 15 knots under mostly overcast skies as flight deck personnel prepared aircraft for take-offs and landings.

Its foghorn blared amid occasional poor visibility to warn fishing vessels and other ships operating in the area.

US officials said the four-day drill, which has angered the North and sparked a protest from China, was planned before the latest attack but designed to send a message of deterrence to Pyongyang.

Washington and Seoul announced a series of joint exercises after a multinational investigation concluded in May that a North Korean torpedo had sunk a South Korean warship with the loss of 46 lives.

Ten other US and South Korean warships are taking part in the latest drill, along with dozens of aircraft.

The North says the exercise brings the peninsula to "the brink of war".

"If the US and South Korean enemies dare to fire one shell in our territory and sea territory, they will have to pay for it," state media said Tuesday.

The drill is taking place off South Korea's southwest coast, far from the flashpoint inter-Korean border.

But China objects to carrier-led exercises in what it sees as its exclusive economic zone in the Yellow Sea.

Asked by pool reporters if any of the drill was taking place in the zone, Rear Admiral Dan Cloyd, commander of the USS George Washington Carrier Strike Group, did not answer directly.

"We will operate in international waters around the world," he said.

"Certainly North Korea has significant military capability, conventional forces, ballistic missile forces," Cloyd said, when asked to describe its potential threat.

"I'm mindful of the capabilities of any potential threats anywhere in the world, of course specifically here in these waters the potential provocations from North Korea.

"And we hope that being here side by side with our Republic of Korea (South Korean) allies will deter them."

The nuclear-powered ship currently has about 5,500 personnel. About 85 F-18 fighter jets are also on board along with other aircraft.

Despite the North's warning of unpredictable consequences if the carrier-led drill went ahead, Commander Pete Walczak said no unusual activities have been detected.

He was speaking in the combat direction centre, which watches for potentially threatening ship or aircraft movements. Screens included maps of the Yellow Sea and the Korean peninsula.

Walczak said the centre can keep watch on aircraft all the way up to northwest China.

"We're looking for bad guys and fighters and submarines and a little bit of the whole shebang," he said.

"We can see all the way up into the North Korea area and we see there's absolutely nothing going on up there," Walczak said. "Nothing. Just blankness."

During the current drill and previous exercises, he said, the situation was always the same.

"A lot of sabre-rattling, fist-shaking, but once our presence is here, reality says that it's really nothing."

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