Thousands of US and South Korean troops, backed by helicopters and jet fighters, have staged a massive, amphibious landing drill.

It's the centrepiece of an annual military exercise, condemned by North Korea.

More than 7500 soldiers, including 3500 marines, 80 aircraft, 30 ships, and scores of armoured vehicles and tanks, have taken part in the drill to secure a bridgehead about 360 kilometres south of Seoul.

The US and South Korea insist the joint exercises are defensive in nature, but North Korea denounces them as provocative rehearsals for invasion.