South Korean officials have responded cautiously to a US proposal to cut by a third its troops based in the country.
Talks on repositioning US forces will continue
Officials said more negotiations were needed, and South Korea needed time to prepare "countermeasures".
China indicated that it welcomed the plan, saying it hoped it would contribute to regional stability.
But South Korean opposition, civic groups and analysts said the relatively quick timescale for the pull-out could jeopardise security.
The US has proposed withdrawing 12,500 of its 37,000 troops by 2006.
The US has said it needs to modernise its forces, but the move may leave some South Koreans feeling vulnerable to North Korea.
"There still need to be negotiations," Defence Minister Cho Young-kil said on Tuesday.
"In response to the US-presented broad idea, our idea is also going to prepare a countermeasure in a broad framework," a government official said on condition of anonymity.
US TROOPS IN SOUTH KOREA
US to pull out 12,500 troops
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US plans to pull remaining troops back from DMZ
"I think the direction of our response will be decided this week after concluding a careful review of the US proposal and collecting various opinions," he said.
The main opposition Grand National Party was less sanguine, calling the US plan "shocking and surprising".
"The number of troops Washington wants to cut came as no surprise, but the timing is rather faster than expected," Professor Kim Tae-hyo at the Institute of Foreign Affairs and National Security told The Korea Herald.
The Pentagon 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.
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.
Plans are already under way to redeploy the US troops which will remain in South Korea, and the two sides held a second day of Future of the Alliance talks on that issue in Seoul on Tuesday.
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.