South Korean and US Marines joined in military exercises in sub-freezing temperatures, including shirtless hand-to-hand combat in the snow, prompting warnings of retaliation from the North over "madcap mid-winter" drills.
More than 300 Marines took part in the training, simulating combat situations on the ski slopes of Pyeongchang, host of the Winter Olympics in 2018, amid speculation of North Korea is testing another intercontinental ballistic missile, in defiance of UN resolutions.
"US marine corps and ROK (Republic of Korea) marine corps partnered together at every level to build the camaraderie and friendship of the two countries' militaries but also to increase our proficiency in the event that we have to fight a war together. So that's what we've been doing for the last few weeks, we partnered up like I said, down to the lowest level so that every marine, US and ROK are familiar with each other in the event that we go to war together," said US Captain, Marcus Carlstrom.
The training began on January 15 and will continue until February 3 in Pyeongchang, about 180 km (115 miles) east of Seoul.
"By strengthening the willpower and combat power with this special winter training, we will infiltrate enemy camp and accomplish our missions without fail whenever an order is given," said South Korean Captain, Kim Jong-bum.
About 28,500 US troops are stationed in South Korea in joint defence with the South's forces against North Korea. Poverty-stricken, reclusive North Korea and the rich, democratic South are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty.
James Mattis, in his confirmation hearing as US defence secretary, described "the Pacific theatre" as a priority and analysts expect new US military spending under President Donald Trump's administration would strengthen America's military presence in Asia.
Topping US concerns in the region are North Korea's nuclear weapons and missile programmes and China's military moves in the South China Sea. The North regularly threatens to destroy the South and the United States.