His remarks came after US special envoy Zalmay Khalilzad was quoted Monday as saying the US and the Taliban have drafted the framework of a deal which could pave the way for negotiations with the Afghan government.

"I'd say really the takeaway right now: it's encouraging," Shanahan told Pentagon reporters as he waited to greet NATO Secretary General Jens Stoltenberg.

Khalilzad has been leading a months-long diplomatic push to convince the Taliban to negotiate with Kabul, but the militants have steadfastly refused, dismissing the authorities as "puppets."

US officials have said President Donald Trump wants to withdraw about half of the 14,000 US troops in Afghanistan.

Shanahan did not provide details about potential US troop withdrawals but, in response to a reporter's question, said he had not been tasked to prepare for a "full" pull-out.

Stoltenberg later said it was too early to speak about the future disposition of NATO forces in Afghanistan.

"We are in Afghanistan to create the conditions for a peaceful negotiated solution," Stoltenberg said.

"We will not leave before we have a situation that enables us to leave or reduce the number of troops without jeopardizing the main goal of our presence -- and that is to prevent Afghanistan from becoming a safe haven for international terrorists."