"Talks resumed this afternoon," the US source said.
The second day of the two sides' ninth round of dialogue had been due to begin on Friday but "both sides decided to postpone it until Saturday", the Taliban's Doha spokesperson Suhail Shaheen said, citing "other engagements".
"We have made progress and are now discussing the implementation mechanism and some technical points," he said on Saturday, referring to discussions that took place on Thursday.
"The agreement will be completed after we agree on these points," he said.
Any deal would be announced before the media as well as representatives from neighbouring countries and China, Russia, and the United Nations, he added.
The US, which invaded Afghanistan and toppled the Taliban from power in 2001, wants to withdraw thousands of troops but only in return for the group renouncing al Qaeda and curbing attacks.
Kabul: carnage in a wedding hall
Washington is hoping to strike an agreement with the Taliban by September 1 -- ahead of Afghan polls due the same month, and US presidential polls due in 2020.
The Taliban claimed on Saturday to have killed seven members of the US military in an attack on a convoy near Bagram airfield north of Kabul.
Colonel Sonny Leggett, a spokesperson for US forces in Afghanistan, said some local businesses had been damaged in the attack but denied coalition or US troops were hurt.
"Rather than taking responsibility for, and apologising for, the harm they bring to innocent Afghans and their businesses, the Taliban spread lies about the attack," Leggett said.
On Wednesday, two US soldiers were killed by small arms fire in Faryab province in northern Afghanistan, the Pentagon said.
Shaheen, the Taliban spokesperson, had said the deaths should have a "positive" impact on the talks in Doha.
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