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.
The intra-Afghan talks have taken on increased significance against the backdrop of the U.S election, with President Donald Trump keen to end the nearly two decade-long U.S. war in Afghanistan and see U.S. troops home by Christmas – well ahead of the proposed withdrawal timeline.
A bipartisan U.S. House of Representatives group wrote to the secretaries of State and Defense – Mike Pompeo and Mark Esper – asking them to explain the U.S. troop withdrawal policy in light of a tweet in which Trump said he wanted U.S. forces home by Christmas.
The four Democrats and two Republicans noted that Khalilzad told lawmakers on Sept. 22 that a final withdrawal would be based on ground conditions, according to a copy of the Oct. 26 letter reviewed by Reuters.
Khalilzad also said that the Taliban had not fulfilled their February accord commitment to break ties with al Qaeda, they noted.
Reporting by Jonathan Landay; Writing by Euan Rocha; Editing by Angus MacSwan and Jonathan Oatis