Acting Defence Secretary Patrick Shanahan said he has been briefed on the talks and described them as encouraging, but he also told reporters his department has not been directed to prepare for a full withdrawal.
Speaking before a meeting at the Pentagon with Mr Shanahan, Nato secretary general Jens Stoltenberg said any discussion about the withdrawal of Nato forces from Afghanistan would be premature.
He said Mr Khalilzad had briefed Nato allies on the talks weeks ago.
"We are in Afghanistan to create the conditions for a peaceful negotiated solution," Mr Stoltenberg said.
"We will not stay longer than necessary, but we will not leave before we have a situation that enables us to leave or reduce the number of troops without jeopardising the main goal of our presence, and that is to prevent Afghanistan from becoming a safe haven for international terrorists once again."