United States' special forces units have arrived in South Korea for a joint military drill with South Korean troops as the allies ramp up combined readiness on the prospect of North Korea's imminent long-range missile launch, the two countries' militaries said Thursday.

The U.S. special operational forces, including those from the airborne 1st Special Forces Group and the 75th Ranger Regiment, came here to train with South Korea's Special Warfare Command and to increase joint special operations readiness and capability, according to United States Forces Korea.

The South Korean Army's special troops kicked off a 10-day winter exercise last week in the mountainous town of Goesan in North Chungcheong Province, which will end on Friday.

With the main goal to train special forces for enemy penetration missions, the exercise has involved drills to land in and take over a key enemy facility as well as winter-survival drills, according to the South Korean Army.

"Sustaining rotational U.S. special operations forces in Korea strengthens the Alliance by ensuring a high level of ready, flexible and agile combined special operation forces," USFK said in a statement. 

It also enables the USFK commander to "leverage special operations forces capability to deter regional asymmetric aggression and maintain peace for the Korean people," it said.

The allies are bracing for North Korea's possible long-range missile launch in the coming weeks, fully deploying their intelligence assets to detect and trace a North Korean missile if it is fired.

North Korea has recently informed international maritime, aviation and telecommunication agencies that it will launch a rocket to put a satellite into orbit, which the outside world suspects is a pretext for the test of a long-range ballistic missile the North is developing in defiance of the United Nations Security Council. (Yonhap)