Both the U.S. and the Taliban have denied that they have reached a final agreement.
"Negotiations will continue today. We are close to an agreement," Taliban spokesperson Suhail Shaheen said Wednesday.
"Negotiations are proceeding. We do not have an estimate for how long it may take to close out the remaining issues," a State Department spokesperson said Tuesday night.
While Khalilzad prepares to brief Afghan President Ashraf Ghani and other officials in Kabul, his team remains behind negotiating with Taliban representatives, two sources said.
After Kabul, Khalilzad will travel to Pakistan and brief leaders there, according to one source, before heading back to Doha, Qatar, where an agreement would be finalized and possibly signed.
For the U.S., a final deal must include a U.S. troop withdrawal; the beginning of intra-Afghan talks with the Taliban, Afghan government and other leaders; a nationwide ceasefire; and a commitment from the Taliban to keep the country from becoming a terror safe haven.
What's still unclear is whether the Taliban will agree to the U.S. keeping counterterror forces in the country, whether they'll agree to a nationwide ceasefire with Afghan security forces and when and at what pace U.S. troops would leave.
The challenge of protecting the American homeland -- what first brought U.S. forces to Afghanistan in October 2001 after the Sept. 11 attacks -- has been at the top of President Donald Trump's mind.
"It's very important that we continue intelligence there in all cases because it is somewhat of a nest for hitting us," Trump said on Aug. 18. "There's a big argument to be made, and I buy that argument."
But the issue has been particularly vexing in negotiations, as the Taliban demands that a U.S. withdrawal mean all American forces.
The top U.S. military official seemed to say on Wednesday that ensuring Afghanistan isn't a terror safe haven would require a U.S. military presence for the time being.
"Could we talk conceptually about a time in the future when the Afghan security forces can deal with security in their country by themselves? You can. But we're not prepared to have a specific conversation about when that may be or what capability will be associated with what operating environment," said Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff. "We will need our interests addressed."
Critics have also challenged how the U.S. will ensure that the Taliban, which maintains ties with al-Qaeda and has battled a growing Islamic State affiliate, will keep the country free of terror groups.
"Any deal, if one is reached, will be so stringently monitored and verified. The agreement we're working on is not based on trust -- it will be based on clear requirements and commitments, subject to our monitoring and verification and will be in sync with the understandings we reach" with the Afghan government, a State Department spokesperson told ABC News on Wednesday.
There are also questions about the Afghan government's capability. Dunford said its security forces, which Khalilzad vowed had continued U.S. support, still require American backing.
"Right now, it's our judgement that the Afghans need support to deal with the level of violence" from the Taliban insurgency, he said, noting that could change if a deal with the Taliban is reached.
U.S. officials are also keenly aware that any deal with the Taliban is just the first step in a long road that next runs through those intra-Afghan talks.
But top officials like Secretary of State Mike Pompeo have not said whether a U.S. withdrawal is entirely tied to the success of those talks, and Trump has made clear his strong desire to begin bringing troops home soon.