KABUL: A fresh round of US-Taliban peace talks will start in Qatar's capital Doha on Saturday, officials said, describing it to be the "most crucial" phase of negotiations to end the 18-year war in Afghanistan.

Senior officials privy to the talks said a peace agreement could be expected at the end of the eighth round of talks and would enable foreign forces to be withdrawn from the war-torn country.

Zalmay Khalilzad, the US peace envoy for Afghanistan who has held a series of meetings with Taliban leaders since last year, reached Doha on Friday night.

"Just got to Doha to resume talks with the Taliban. We are pursuing a peace agreement not a withdrawal agreement," Khalilzad wrote on Twitter.

"A peace agreement that enables withdrawal. Our (US) presence in Afghanistan is conditions-based, and any withdrawal will be conditions-based," he said, adding the Taliban are signalling they would conclude an agreement.

"We are ready for a good agreement."

Two sources with knowledge of the talks said an agreement on the withdrawal of foreign forces in exchange for security guarantees by the Taliban is expected before August 13.

About 20,000 foreign troops, most of them American, are now in Afghanistan as part of a US-led NATO mission to train, assist and advise Afghan forces.

The hardline Islamist Taliban group now controls more territory than at any point since the United States bombed them out of power in 2001.