Turkey's military said it had launched the land incursion backed by fighter jets on Thursday evening to hunt down Kurdish PKK fighters in Iraq's largely autonomous region of Kurdistan.
Rear Admiral Gregory Smith, a U.S. military spokesman in Baghdad, said the operation was understood to be of "limited duration" and specifically targeted at PKK rebels.
"Turkey has given its assurances that it will do everything possible to avoid collateral damage to innocent citizens or Kurdish infrastructure," Smith said in a statement.
Smith said the United States continued "to support Turkey's right to defend itself from the terrorist activities of the PKK".
His statement said the U.S. position was to encourage Turkey to use all available means to resolve the issue, including diplomacy and close cooperation with Iraq's government.