US and South Korean troops have commenced a major annual air, ground, naval and special operations support exercise amidst North Korean threats.
The Key Resolve / Foal Eagle drills will involve 7,700 US troops and 10,000 South Korean service members, as well as participation from UN nations including Australia, Canada, Denmark, France and Great Britain.
Conducted to maintain South Korea's readiness, the exercise will train the forces in responding to potential national security threats.
Pentagon spokesman Navy captain Jeff Davis said: "As you know, we continue to ask [North Korea] to refrain from provocative actions and statements that only serve to aggravate tensions.
"We closely monitor activities [in North Korea], and we're always ready to act in support of our Republic of Korea allies."
In a separate development, both countries are considering the viability of a terminal high-altitude area defense system (THAAD) operated by US Forces Korea.
The THAAD missile system is designed to intercept and destroy short, medium, and intermediate ballistic missiles using hit-to-kill technology.
It is claimed to be the only system capable of engaging ballistic missiles at both endo and exo-atmospheric altitudes.
Since 2005, the THAAD development programme has completed 13 flight tests, and has recorded 11 successful intercepts in 11 attempts. The fifth of seven programmed THAAD batteries was activated by the US Army last year.