The seventh round of peace talks between the United States and Afghanistan's Taliban that are ongoing in Qatar are the "most productive" so far, US envoy Zalmay Khalilzad said Saturday.

The insurgents have been meeting with the US envoy in Doha to hammer out a deal that would see the US military quit Afghanistan in return for various guarantees.

"These six days have been the most productive of the rounds we've had with the Talibs," Khalilzad said in Qatar.

"Essentially the four items we have been talking about ever since we started [are] terrorism, withdrawal of foreign troops, inter-Afghan negotiations and dialogue, and cease-fire.

"For the first time I can say we have had substantive discussions, negotiations, and progress on all four issues."

The envoy added that direct US-Taliban talks seeking to end nearly 18 years of war would pause for a separate two-day meeting of Afghan representatives and resume on Tuesday.

Suhail Shaheen, the spokesman for the Taliban's office in Qatar, said they were pleased with the US discussions so far.

"We are happy with progress and hope the rest of the work is also done. We have not faced any obstacles yet," he tweeted.

About 60 Afghan delegates were expected to attend the so-called intra-Afghan dialogue on Sunday and Monday, including political figures, women and various Afghan stakeholders.

The Taliban, who have steadfastly refused to negotiate with the government of President Ashraf Ghani, have stressed that those attending the intra-Afghan talks will only do so in a "personal capacity."

The US does not participate in the intra-Afghan talks, which envoy Khalilzad said would not be on the "Taliban's terms." The US envoy likewise emphasized that those attending the intra-Afghan dialogue would be there only in their personal capacity. 

"Senior [Afghan] government people are here - the president's senior advisers... everyone, as far as the Afghan dialogue is concerned, is here in their personal capacity," said Khalilzad.

Observers have criticized the US for allowing its own talks with the Taliban to proceed without the formal participation of the Afghan government, which the Taliban label Washington's puppet.