South Korea and the United States kicked off their annual joint exercise Ulchi Freedom Guardian (UFG) 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 (CFC) said the drill will run to September 2, and involve about 75,000 troops, including 25,000 from the US side, according to (Yonhap) news agency.

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, Yonhap reported.

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 October 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 US commitment to the Alliance,” it added.

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

South Korea and the US “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 (KPA) said.