NEW YORK—The United Nations said on Thursday it is particularly concerned about a shift in fighting in Afghanistan to urban areas, warning that if a Taliban offensive reaches the capital Kabul it would have a “catastrophic impact on civilians.” The Taliban terrorist group claimed control over the third largest city, Herat, on Thursday and appeared close to capturing Kandahar, the second largest city. Taliban now control about two-thirds of Afghanistan. “It is clear that urban fighting in the city of the size of Kabul would have catastrophic impact on civilians and we very much hope that this does not happen,” U.N. spokesman Stephane Dujarric told reporters. Dujarric also said any investigation into civilian deaths would have to be impartial and independent from the warring parties. The United Nations said more than 1,000 civilians had been killed in the past month. In a statement on Wednesday the Taliban denied killing civilians …