South Korean and U.S. troops on Friday (April 8) conducted a river-crossing drill in Yeoncheon, just a few miles from the demilitarised zone separating the two Koreas, to enhance their combined military operations.

About 400 U.S. soldiers and 100 South Korean soldiers participated in the exercise as four tanks and twelve Bradley fighting vehicles from the U.S. were mobilized for the drills, said a U.S. military official.

Tanks and Bradley fighting vehicles crossed the Imjin River after soldiers constructed temporary floats and a bridge.

"Our mission over here is to deter North Korea's aggression with our partner South Korean allies across multiple formations and help to sustain our alliance here on the peninsula," said U.S. Army Major, Ed Arntson, a Brigade Operations Officer from the 1st Armored Brigade Combat Team.

The drill was not part of the annual joint U.S.-South Korean drills which involves about 17,000 American troops and more than 300,000 South Koreans.

Tension in the region is high as South Korea and the United States began large-scale military exercise last month in a test of their defenses against North Korea, which called the drills "nuclear war moves" and threatened to respond with an all-out offensive.

There are 28,500 U.S. troops stationed in the South as part of combined defence with the South's military of more than 600,000. The North has an army of 1.2 million.

North and South Korea are technically at war since the 1950-53 Korean War ended in a truce without a peace treaty.