The US military will deploy 800 additional troops from an armored unit to South Korea to bolster its forces in case of a crisis with North Korea, the Pentagon said Tuesday.

The US Army soldiers, armored vehicles and tanks from the 1st Battalion, 12th Cavalry Regiment will be stationed at camps Hovey and Stanley near the demarcation line with the North starting next month, military officers said.

The rotational deployment is part of a strategic shift towards the Asia-Pacific region "and allows for greater responsiveness to better meet theater operational requirements," the Pentagon said in a statement.

"This is a plus-up," spokesman Colonel Steven Warren told reporters, confirming that the US military presence would expand under the decision.

The move "is part of our rebalance to the Pacific," he said.

The US military already has 28,500 troops on the ground in the South and the decision to send reinforcements appeared to signal Washington's concern over possible provocations from volatile North Korea.

But Warren said the deployment had "been long planned."

The unit includes 40 Abrams M1 tanks and 40 Bradley armored fighting vehicles, he added.

Under a treaty with Seoul, the US military commander would lead both the American contingent and South Korea's 640,000-strong force in case of a war with the North.

During peacetime, each side maintains operational command of their own troops.

South Korea agreed to take over wartime operational command of all troops starting in 2015, after delaying a previous target date in 2012.

Tensions with North Korea have prompted Seoul to reconsider the plan, and the South has asked Washington to review the timing of the scheduled transition.

The "combat ready" cavalry unit will stay in South Korea for a nine-month tour but will leave its armored vehicles and tanks behind for troops that follow them, the Pentagon said.

The announcement came a day after Defense Secretary Chuck Hagel hosted South Korea's Foreign Minister Yun Byung-Se for talks at the Pentagon.

"The two discussed the importance of maintaining a robust combined defense of the Korean Peninsula as a strong deterrent against provocations from North Korea," spokesman Rear Admiral John Kirby said.

South Korea and the United States have called for vigilance on the peninsula after North Korean leader Kim Jong-Un ordered the execution of his uncle, Jang Song-Thaek, for an alleged plot.