KABUL (Reuters) - A U.S. investigation into reports that at least 13 civilians were killed during an operation in the northern Afghan city of Kunduz last week found no evidence of any civilian casualties, a statement from the U.S. military in Kabul said on Tuesday.  United States Forces   Afghanistan (USFOR-A) has investigated allegations of civilian casualties in Kunduz province during the period of November 3 and 4; no evidence of civilian casualties has been found,  the statement said. Officials in the Chahardara district outside Kunduz city said 13 people in two villages had been killed by U.S. air strikes in the area, while some reports said as many as 65 people had been killed. However, other Afghan civil and military officials denied the reports and said civilians had been evacuated before a combined operation in the area began. They said dozens of Taliban insurgents had been killed.  We can confirm operations occurred in this area and numerous enemy combatants were killed, as also confirmed by Kunduz Governor Omarkhail and Ministry of Defense Spokesman Major General Dawlat Waziri,  the U.S. statement said.  The USFOR-A investigation was conducted independently and concluded that there were no civilian casualties. Specifically, no hospitals or clinics in the local area indicated treatment of people with wounds from armed conflict,  it said. The issue of civilian casualties has taken on increasing sensitivity as the United States has stepped up air strikes against the Taliban as part of a more robust strategy aimed at breaking the stalemate with the insurgents. United Nations figures from last month showed a 52 percent increase in civilian casualties from air strikes in the first nine months of the year from the previous year, with 205 killed and 261 wounded. 