The United States is sending an additional army combat force of 800 soldiers to South Korea with tanks and armoured troop carriers.
The US also pledged on Tuesday to continue to modernise the military capability of its ally to face any threat posed by North Korea.
The announcement came as Secretary of State John Kerry met his South Korean counterpart, Yun Byung-se, in Washington DC.
Kerry reiterated that the US would maintain its nuclear defence for South Korea and would not accept North Korea as a nuclear state.
The US already has about 28,500 troops in South Korea as an expression of commitment to its defence against North Korean aggression.
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An armistice, but not a peace treaty, halted the 1950-1953 Korean War.
Concern over the North's development of weapons of mass destruction has intensified in past year, as it has launched a long-range rocket and conducted its third nuclear test explosion.
"We remain fully committed to the defence of the Republic of Korea, including through extended deterrence and putting the full range of US military capabilities in place. We will continue to modernise our capabilities so that we are prepared to face any threat," Kerry said after meeting Yun.
A Pentagon announcement said the 1st Battalion, 12th Cavalry Regiment from the 1st Cavalry Division at Fort Hood, Texas, will deploy to two locations in South Korea on February 1.
A Pentagon spokesman, Army Col. Steve Warren, said the increase in troop strength and firepower had been in the planning stages for more than a year and is part of a "rebalance" of US military power toward the Asia-Pacific region.
