The US request came on Sunday evening as both sides prepared to open the two-day Future of the Alliance talks, said Kim Sook, head of the South Korean Foreign Ministry's North American bureau on Monday.
The US delegation, led by Assistant Secretary of Defence Richard Lawless, said Washington wanted to withdraw some 12,500 US troops by December 2005, Kim said.
The figure would include about 3600 already slated to be redeployed this summer from South Korea to Iraq.
Kim said officials at the South Korean National Security Council, Defence Ministry and Foreign Ministry would review the proposal before giving a response: "We'll formulate a position and then notify the United States."
Although communist North Korea's 1.1-million-strong armed forces dwarf the US contingent, any reduction is closely watched because its symbolic deterrent value outweighs its numerical strength. Ally South Korea has 690,000 troops.
Washington announced late last year that it aimed to transform its forces deployed worldwide using advances in military technology and smaller more mobile units to better respond to different types of security needs.
Seoul says move will help it make
military more self-sufficient
Kim said the United States welcomed Seoul's plans for a
more independent defence and would actively help South Korea as it upgrades its military to be more self-sufficient.
South Korean and US officials have said the redeployment
to Iraq, and any future changes to the US military in the
South, would not weaken their combined defence against the
The United States has earmarked $11 billion for force
upgrades, while South Korea has discussed plans to increase
Because the 1950-53 Korean War ended in a truce rather than a peace treaty, the two Koreas are technically still at war.
North Korea's military is the world's fifth largest and the army plays a dominant role in the country's politics and economy. South Korea's is the sixth-largest military in the world.