The UN chief urged US and NATO-led forces in Afghanistan on Wednesday to prevent civilian casualties in their operations, echoing concerns of President Hamid Karzai who has increasingly voiced his anger over the issue. Hundreds of Afghan civilians have died in attacks by the Taliban - but also in operations by US and NATO forces fighting the insurgents. Speaking alongside UN Secretary-General Ban Ki-moon - whose trip to Kabul was unannounced - Karzai said he has publicly complained over civilian casualties in order to pressure US-led forces into changing their tactics. Ban said he shared Karzai's concerns, calling for closer civilian-military cooperation and urging foreign troops to ensure that civilians are not killed. He said there have been "many tragic incidents." The UN chief's visit coincides with a renewed focus on the fight against militancy in the country by President Barack Obama's administration. It also comes at a time when foreign and Afghan forces are trying to reverse gains made by the Taliban, which has made a comeback after its initial defeat following the 2001 US invasion.