The US and South Korean militaries have started the annual, joint-staff exercise code-named Exercise Ulchi Freedom-Guardian (UFG), in Seoul, South Korea.
A South Korean Defence Ministry statement was reported by Yonhap News Agency as saying that around 50,000 South Korean and 30,000 US personnel are participating in the exercise, which aims to evaluate and improve their joint defence posture against North Korean provocations.
For the first time, the 12-day drill will see the two allies implementing their customised deterrence plan against North Korea's nuclear threats and weapons of mass destruction.
Meanwhile, North Korea has threatened to launch 'pre-emptive attacks' in response to the exercise, which concludes on 29 August.
In a statement carried by the Korean Central News Agency, a North Korean military spokesperson said: "We again declare that we will open the strongest, most merciless pre-emptive attacks of our own style at any time since the US and the South have threatened to deploy their customised deterrent strategies for their exercises."
In response, an unnamed South Korean Defence Ministry official said: "During the UFG period, we will further strengthen reconnaissance activities to maintain a tighter defence posture against North Korea."
North Korea launched three short-range rockets off its east coast on 14 August, shortly before Pope Francis arrived in Seoul on his first visit to Asia.
UFG is the world's largest computer-simulated drill aimed at validating the South Korea-US alliance's operational readiness to respond to threats across a range of potential conflicts, including North Korean provocations.
Conducted since 1976 across the US Combined Forces Command and South Korean military installations, the drills, previously called Ulchi Focus Lens, are connected by communications networks that reach to US military headquarters worldwide.
North Korea has routinely criticised the exercise as a rehearsal for war, despite assurances from the US and Seoul that it is purely defensive and aims to test their combined readiness.
Image: US and South Korean personnel during Ulchi Freedom Guardian 2010. Photo: courtesy of Sgt. 1st Class Raymond Piper.