Taliban militants killed 30 security forces in an ambush on two checkpoints in the far-western province of Badghis early on Wednesday, the provincial governor said, their first major attack since the their Eid ceasefire.
The Taliban's three-day ceasefire ended on Sunday.
Before the latest attack, prospects had risen for negotiations between the Taliban and the United States after Afghan President Ashraf Ghani called the Eid ceasefire and allowed militants to roam into cities in a gamble to encourage peace talks.
The Taliban, ousted from power in 2001 by US-led troops, insist that any negotiations with what it calls the "puppet" Afghan government on a peace plan can begin only after talks with the United States about withdrawing foreign forces.
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Analysts and western diplomats said Ghani's offer to hold unconditional peace talks set the stage for US officials to open back-channel negotiations with the Taliban.
"Ghani has done his bit," said Thomas Ruttig, co-director of Afghanistan Analysts Network, an independent think tank.
"It is now for the US to cut through this blockade," he said, although that would be a departure from US policy that talks to end the 17-year-old war must be wholly Afghan-led.
US Secretary of State Mike Pompeo appeared ready to tweak the policy when he welcomed Ghani's 10-day extension of a ceasefire that is currently due to end on Wednesday. The Taliban said its ceasefire ended on Sunday.
"As President Ghani emphasised in his statement to the Afghan people, peace talks by necessity would include a discussion of the role of international actors and forces," Pompeo said. "The United States is prepared to support, facilitate and participate in these discussions."