NEW YORK, Aug 12 (Reuters) - 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 claimed control over the third largest city, Herat, on Thursday and appeared close to capturing Kandahar, the second largest city and the spiritual home of the Taliban, which 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 and suggested a U.N. team, accompanied by them, conduct an inquiry. The Islamist militants proposed that a team made up of the United Nations, Red Cross and other international aid groups accompany Taliban representatives "to conduct an impartial and independent investigation into the latest events." The Taliban has stepped up its campaign to defeat the U.S.-backed government since April as foreign forces complete their withdrawal after 20 years. In the first six months of 2021, the United Nations said 5,183 civilians had been killed or injured, blaming the Taliban for 39% - 699 deaths and 1,345 wounded - and Afghan government forces for 23% - 378 deaths and 828 wounded. Peace talks between the Afghan government and Taliban negotiators started last year in the Qatari capital of Doha, but have not made any substantive progress. "We are continuing to believe that there is a political solution that can be had. This doesn"t mean that we are also blind to what is going on in the on the ground," Dujarric said.