The Annual US-South Korean "Key Resolve" military exercises begins.
North Korea see the drills a provocation of war while the allies state the exercises are intended to build upon a strategic alliance.
This year's exercise comes days after North Korea launched its third nuclear test, provoking international condemnation.
The Korean peninsula has been under dispute since the 1953 Korean civil war. Although the war ended in a truce, no peace treaty has been signed as yet.