US and South Korean warships staged anti-submarine drills Monday as part of a major naval exercise intended to send a warning to North Korea despite its threats of nuclear retaliation.

The two allies, who accuse the North of sending a submarine to torpedo a South Korean warship, have assembled about 20 ships including the 97,000-ton carrier USS George Washington, 200 aircraft and 8,000 personnel.

Four F-22 Raptor stealth fighters are flying in and around Korea for the first time to show Washington's strong commitment to deter and defeat any provocative acts, Lieutenant General Jeffrey Remington, commander of the US 7th Air Force, told reporters at Osan air base.

Seoul and Washington say the four-day exercise which began Sunday -- their biggest for years and the first in a series -- is designed to stress that future attacks will meet a decisive response.

The United States has also announced new sanctions to punish the North for the sinking and push it to scrap its nuclear weapons programme.

The communist North denies responsibility for the attack on the South's corvette in March which cost 46 lives. It describes the drill named "Invincible Spirit" as a rehearsal for war.

Monday's manoeuvres "focus on better detecting intrusions by an enemy's submarines and attacking them", a spokesman for the South's Joint Chiefs of Staff told reporters.

The South's military came in for strong criticism for failing to detect the alleged submarine attack near the disputed Yellow Sea border.

Hundreds of sailors lined the flight deck of the George Washington as 28 planes staged a ceremonial flypast in several waves, according to a pool report from the ship.

The exercise is being held in international waters in the Sea of Japan (East Sea), 200 km (125 miles) south of North Korean waters.

But the North's powerful National Defence Commission said Saturday the country's army and people "will legitimately counter with their powerful nuclear deterrence the largest-ever nuclear war exercises to be staged by the US and the South Korean puppet forces".

At a meeting in Pyongyang Monday attended by senior party, army and state officials, armed forces minister Kim Yong-Chun said the North would strengthen its nuclear deterrent "in a new advanced manner" to cope with the increasing US nuclear threat.

Kim, quoted by the official news agency, gave no details.

Last September the North announced it had reached the final stage of enriching uranium, a second way of making nuclear bombs in addition to its plutonium-based operation.

Should the US and South Korea ignite a new war, the North "will mobilise the tremendous military potential including its nuclear deterrence for self-defence and thus wipe out the aggressors and blow up all their strongholds," Kim said.

The meeting was called to mark the 57th anniversary on Tuesday of the armistice which ended the three-year Korean War, in which Pyongyang says it was victorious.

Rear Admiral Dan Cloyd, commander of Carrier Strike Group Five, told a pool reporter the war games are purely defensive in nature.

"Our intent is to improve defence capabilities in areas such as anti-submarine warfare, air defence and anti-surface warfare," Cloyd said.

"Our intent is not to provoke reactions from any nation, be it North Korea, or any other here in the Western Pacific region."

In response to Beijing's protests, the current exercise was switched from the Yellow Sea separating China and Korea to the other side of the peninsula.

But officials said future drills would be held in the Yellow Sea as well as the Sea of Japan.