Seoul already has cut 240 personnel in recent months to consolidate its military units in Iraq, officials said. There would be a further cut of about 30 until August, officials said on Thursday.
"There were factors that called for both an increase in some functions and a cut in others," a defence ministry official said.
But, she added: "There is no plan to reduce the overall scale of the deployment."
South Korean media reports quoted US sources as saying the cuts had involved more than 500 military personnel and that Washington had not been notified of the move.
Seoul officials denied the reports.
Even with the reduction, South Korea still would have the third-largest foreign military contingent in Iraq after the United States, which has about 150,000 personnel, and Britain at about 8600, another official said.
South Korean President Roh Moo-hyun, who pledged troops for Iraq despite strong opposition at home, said in January that the troops would remain until goals set by Washington and its allies had been accomplished.