South Korea and the United States kicked off their annual joint exercise Ulchi Freedom Guardian on Monday to maintain their combined forces' readiness against North Korea's potential aggression amid Pyongyang's threats to launch a pre-emptive nuclear strike.

The allies' Combined Forces Command said the drill will run to Sept. 2, and involve about 75,000 troops, including 25,000 from the U.S. side.

As the countries opened the command and control exercise earlier in the day, they told North Korea of the drill's schedule and its non-offensive nature through a communication channel in the inter-Korean truce village of Panmunjeom, according to the CFC.

For this year's exercise, nine member countries of the United Nations Command based in South Korea will join the computerized military exercise, including Australia, Canada, France and Britain.

The Neutral Nations Supervisory Commission, the armistice watchdog, will also participate in the drill as an observer in charge of looking into whether it is in compliance with the armistice agreement between the two Koreas, according to the CFC.

"Training exercises like UFG are carried out in the spirit of the Oct. 1, 1953, ROK-U.S. Mutual Defense Treaty and in accordance with the Armistice," the CFC said in a statement. "These exercises also highlight the longstanding military partnership, commitment and enduring friendship between the two nations, help to ensure peace and security on the peninsula, and reaffirm the U.S.

commitment to the Alliance."

As the exercise began, North Korea threatened a pre-emptive nuclear attack on the allies.

South Korea and the U.S. "should bear in mind that if they show the slightest sign of aggression on (DPRK's) inviolable land, seas and air ..., it would turn the stronghold of provocation into a heap of ashes through Korean-style preemptive nuclear strike," the General Staff of the (North) Korean People's Army said.

South Korea's government has warned over the weekend that Pyongyang is likely to carry out military provocations during or after the exercise which closely followed Seoul's confirmation last week of a London-based senior North Korean diplomat's defection to the South.

North Korea customarily reacts with anger and military threats to such joint military exercises which they say are a rehearsal for invading North Korea. (Yonhap)