Pentagon officials on Monday confirmed the first reduction in US force levels on the Korean peninsula since the early 1990s.
The decision to remove troops defending South Korea dramatically underscored the strains that have been placed on the US army as it fights Iraqi insurgents.
But Deputy Under-Secretary of Defence for Asia Pacific policy, Richard Lawless, said the relocation of the 2nd Brigade, 2nd Infantry Division to Iraq was part of a global realignment of US forces.
He said it would result in "no diminution of capabilities either in the region or on the Korean peninsula."
No decision had been made on whether the brigade would return to South Korea after its year-long tour in Iraq.
The move will reduce the number of US ground troops in South Korea to 34,000, the official said, the first time the level has fallen below 37,000 since the early 1990s.
Asked whether the move might be read by North Korea as a sign the US was tied down in Iraq, a senior military official said: "That would be a misperception, let's put it that way."
President George Bush discussed the plans on Monday with South Korean President Roh Moo-Hyun and Japanese Prime Minister Junichiro Koizumi, both of whom expressed their "support and understanding," the White House said.