The US and South Korea have launched a new round of military exercises involving tens of thousands of troops, despite continued threats of retaliation from North Korea.
The 10-day computerised drills involving around 30,000 US troops and 56,000 South Korean troops began on Monday and is one of a series being staged by the South - either alone or jointly with the United States - in the aftermath of the sinking of one of its warships in March.
On Sunday, Pyongyang warned that its army and people would "deal a merciless counterblow" to the United States and South Korea over the war games.
In a message posted on a US military website, General Walter Sharp, who heads some US troops based in the South, described the exercise as "one of the largest joint staff directed theatre exercises in the world".
The first joint US-South Korean military exercises began last month in an operation code-named "Invincible Spirit".
About 8,000 US and South Korean troops, 20 ships and submarines and 200 aircraft took part in the drill, which the US and South Korea have said is aimed at curbing the North's "aggressive" behaviour.
The sinking of the Cheonan frigate in March with the loss of 46 South Korean sailors raised tensions across the Korean peninsula, with a multinational inquiry determining that it was caused by a North Korean torpedo.
The North has vehemently denied it was involved in the sinking and threatened retaliation for this week's military exercises, which it branded preparations for a full-scale "military invasion".
The Korean peninsula remains in a technical state of war because the Korean War ended in a truce, not a peace treaty.