US and South Korean warships staged anti-submarine drills yesterday 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 on Sunday and ends tomorrow – is their biggest for years and the first in a series – and 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 kilometres south of North Korean waters.

But the North’s powerful National Defence Commission said on 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”.