South Korean and US troops are taking part in joint military drills which will last for 11 days, ending on 30 August.
Here, South Korean security forces are seen taking part in an anti-terror and anti-chemical drill session in Seoul, the capital.
These exercises, which feature drills including the handling of chemical and biological attacks, form part of a regular joint exercise between US and South Korean soldiers to prepare for potential emergencies on the volatile Korean Peninsula.
The Ulchi Freedom Guardian drills are computer-simulated war games involving 30,000 American and 50,000 South Korean troops.
North Korea, with which the South is still technically in a state of war following the 1950-1953 conflict, on Tuesday criticized the military drills but in milder language than that previously seen from the regime of Kim Jong-un. This is being seen as a sign of a slight thawing in tensions between the two sides.
Pyongyang typically uses warlike rhetoric to condemn the South Korea-US exercises, which it has in the past described as a rehearsal for an invasion.