(Reuters) - The U.S. envoy for Afghanistan said on Tuesday that violence there was too high and the Kabul government and Taliban insurgents must work harder toward forging a ceasefire at their peace talks.Special Representative Zalmay Khalilzad made his comments ahead of flying to Doha to meet with the two sides. “I return to the region disappointed that despite commitments to lower violence, it has not happened. The window to achieve a political settlement will not stay open forever,” he wrote in a tweet. The talks between a government delegation and the Taliban have been going on in Doha since mid-September, but progress has been slow and diplomats and officials have warned that rising violence back home is sapping trust. The sides are often at odds on even the most basic issues. “The sides must move past procedure and into substantive negotiations,” Khalilzad’s office said in a statement on Tuesday. There needed to be “an agreement on a reduction of violence leading to a permanent and comprehensive ceasefire,” he said. Last week, Khalilzad said he had struck an agreement with the Taliban to “re-set” their commitments under a troop withdrawal deal and reduce the number of casualties in the conflict. The deal in February between the United States and the Taliban paved the way for foreign forces to leave Afghanistan by May 2021 in exchange for counterterrorism guarantees from the Taliban, who agreed to negotiate a permanent ceasefire and a power-sharing formula with Kabul.