KABUL - Afghan President Hamid Karzai on Tuesday condemned a NATO operation that he said killed four children in the country's east, a claim the coalition said was possible.The operation took place in the Baraki Barak district of Logar province on Saturday, a statement released by Karzai's office said, adding: "It resulted in the killing of four innocent children" who were tending to livestock.Civilian casualties have been a major source of friction between Karzai's government and the US-led NATO forces in Afghanistan."Despite repeated pledges by NATO to avoid civilian casualties, innocent lives, including children, are still being lost," Karzai said in the statement.A spokeswoman for NATO's International Security Assistance Force (ISAF) said it was aware of possible ISAF-related civilian casualties from the October 20 operation.