US Marine forces have been flexing their muscles during assault drills with South Korea in a massive display of joint fire power.

On Wednesday morning, thousands of troops and high-end weaponry were used to practice an amphibious assault, which took the form of a maneuver offensive, to claim territory rather than protect it.

The commander of the 2,200 US Marines involved in Exercise Ssang Yong in Pohang, on the eastern coast of South Korea, said it was not meant to aggravate the opposition.

It comes days after North Korea's saber-rattling display of a tactical nuclear warhead.

North Korea has been strengthening a ballistic missile arsenal on the stated premise that it requires it to stave off an attack by US and South Korean forces.

Washington and Seoul have been displaying their firepower through a growing number of exercises, which the two allies claim are of a defensive nature.

'I don't think we're doing anything different or odd,' said Col. Samuel Meyer, commander of the 13th Marine Expeditionary Unit.

The exercise, though, shows the integrated firepower of the US and South Korea in full force. 

Seoul's Marines landed on the sands of Pohang in groups of 23-ton amphibious assault vehicles. 

As the South Korean Marines advanced to a tree line behind the beach, massive US Navy hovercraft, known as LCACs, came shortly afterwards.

LCACs are high-speed and fully amphibious vehicles transporting, ship-to-shore and across the beach, personnel, weapons, equipment, and cargo of the assault elements of the Marine Air-Ground Task Force. 

During the drills, attack helicopters and planes stalked the skies, including Osprey transports and F-35B stealth fighters, 10 of which were embarked aboard the amphibious assault ship USS Makin Island, 30 miles offshore.

The V-22 Osprey is a multi-engine, dual-piloted, tilt-rotor aircraft designed for combat, combat support, combat service support, and Special Operations missions worldwide. 

The F-35B Lightning II is the Marine Corps variant of the Joint Strike Fighter and features a vertical lift fan and pivoting engine nozzle to deliver vertical landing and short takeoff capability to expeditionary airfields. 

'This is the 70th anniversary of this exercise. It's not new,' Meyer said, in response to claims that the US and South Korea are forcing North Korea to build up its nuclear program as deterrence.

'This is routine. We're just getting back to the routine, based on what we saw and experienced,' he added.