The US is planning to move some of its troops from South Korea to Iraq, according to the Seoul government.
The US currently has 37,000 troops in South Korea
South Korea's foreign ministry said the two governments were currently discussing the details of such a move.
A ministry official said the US was planning to transfer a full brigade of 4,000 soldiers currently stationed near the North Korean border.
The US has had to reverse plans to reduce troop numbers in Iraq because of the escalating violence there.
The South Korean official, Kim Sook, said Seoul had been informed of the troop moves on Monday, and had expressed its understanding.
"The US explained that it is inevitable to move part of US Forces Korea to Iraq for the sake of a successful hand-over of sovereignty to Iraq," Mr Kim said at a press briefing.
The US currently has some 37,000 troops based in South Korea under an agreement dating back to the Korean War 50 years ago.
There are about 130,000 US troops occupying Iraq, with an additional 20,000 from other nations.
The US is planning a major realignment of its forces in East Asia but says it remains fully committed to the defence of South Korea.
However, the BBC's Charles Scanlon in Seoul says the withdrawal of 4,000 men would significantly weaken the strength of the Second Infantry Division - the main US fighting force in South Korea.
The division currently has 14,000 soldiers stationed near the border with North Korea.
Shia militiamen in Iraq have risen up to challenge the US occupation
Our correspondent adds that the plan could affect the South Korean government's promise to send its own troops to Iraq.
Seoul has delayed sending about 3,000 troops because of the worsening security situation in Iraq and growing opposition at home.
The recent withdrawal from Iraq of contingents from Spain, Honduras and the Dominican Republic has added to the pressure on the US to keep troop strength up.
The US-led coalition in Iraq has been facing growing resistance, as it prepares to hand over sovereignty