The United States has proposed withdrawing 12,500 of its 37,000 troops stationed in South Korea by 2006.
The US wants to modernise its military alliance with the South
US officials told South Korean counterparts of the plans ahead of talks in Seoul on troop movements.
The reduction would include 3,600 troops which Washington has already earmarked for redeployment to Iraq.
The US has said it needs to modernise its forces, but the proposed speed and scale of the move may leave the South feeling vulnerable to North Korea.
"US officials told us last night that under their Global Defence Posture Review they are planning to reduce the number of U.S. troops here by 12,500 by the end of December 2005," said Kim Sook, head of the South Korea Foreign Ministry's North America bureau.
The Pentagon says the reduction is part of world-wide changes designed to make the US military more flexible.
US TROOPS IN SOUTH KOREA
US to pull out 12,500 troops
Significance largely symbolic: S Korea has 690,000 troops, N Korea 1.1m
US plans to pull remaining troops back from DMZ
It says its commitment to defend the South will not be affected and the use of longer range weapons and better technology will compensate for the reduction in numbers.
South Korea's President Roh Moo-hyun - who favours less dependence on US forces - has also tried to dispel fears over security.
"The concepts of self-defence and an alliance (with the US) can complement each other," he said in a televised speech on Sunday for the country's Memorial Day for war dead.
But the South does not want big changes until the resolution of the dispute over North Korea's development of nuclear weapons, says the BBC's correspondent in Seoul, Charles Scanlon. It would prefer the pull-out to happen gradually over 10 years.
Mr Kim said officials would review the proposal before responding. "We'll formulate a position and then notify the United States," he said.
Plans are already under way to redeploy the US troops which will remain in South Korea, and the two-day Future of the Alliance talks which began on Monday are mainly focused on that.
At present nearly half the 37,000 US troops in South Korea are stationed north of Seoul, a throwback to the 1950-53 Korean war.
Their forward position puts them in range of North Korean artillery, and US officials have said that pulling troops back south of the capital would strengthen the military's hand.
The US also wants to move the main US army headquarters from its current location in central Seoul to free up money for better military technology and infrastructure.