South Korea will create a combined army unit with the US, reportedly tasked with destroying North Korea's weapons of mass destruction in the event of an all-out conflict, it announced on Thursday.

The mechanised unit led by a US major general will be set up in the first half of next year, the South's defence ministry said, as part of elaborate preparations for any future war between the two Koreas.

"It will be the first combined 'field combat' unit to carry out wartime operations," a defence ministry spokesman said without elaborating on its mission.

If war breaks out, the unit would be tasked with eliminating weapons of mass destruction in the nuclear-armed North, Yonhap news agency said.

The contingent would have a joint office of US and South Korean staff in Uijeongbu, north of Seoul, where the US 2nd Infantry Division guards a strategically important area as a deterrent to an invasion by North Korea.

In the event of a full-scale conflict, the combined unit would absorb a mechanised South Korean brigade and forces from the US division, which is armed with helicopters and other advanced weapons.

Because the Korean conflict ended in an armistice instead of a peace treaty, the two Koreas are still technically at war.

Nearly 30,000 US troops are stationed in the South under a bilateral military accord.

If fresh hostilities broke out, the US commander in South Korea would assume control of the South's 640,000 troops.

A transfer of command in a wartime setting was set for 2015, which would allow South Korea to control its own troops, but the South wants a postponement citing the increased threat from North Korea's nuclear and missile development.

In June, North Korea announced the successful test of new high-precision, tactical guided missiles.

In the following months the North conducted a series of missile and rocket tests, raising tensions on the Korean peninsula.

Meanwhile, South Korea and the US held an annual military drill in August despite condemnation by North Korea, which had threatened a "merciless" retaliatory strike.

The drill simulated the response to a nuclear attack threat for the first time, according to the South's defence ministry, using a strategy of "tailored" deterrence developed last year in the course of joint defence talks.

UN resolutions bar Pyongyang from conducting any launches using ballistic missile technology. But the North has defended its missile launches as a legitimate exercise in self-defence.