There was no immediate claim of responsibility for the suicide attack in Jalalabad city, outside the office of the Nangarhar provincial governor. The Daesh terror group, which was not part of the truce, had claimed an even deadlier blast near the city a day earlier, AFP reported.
Further dampening hopes for peace after jubilant scenes during the ceasefire over the Muslim holiday, the Taliban announced they would not be extending the truce beyond Sunday night.
Nangarhar provincial health director Najibullah Kamawal put the death toll from Sunday's blast at 18 with 49 wounded.
"Some of the wounded are in a serious condition," Kamawal added, suggesting the death toll could rise.
Governor's spokesman Attaullah Khogyani the bomber on foot blew himself up among a crowd of Taliban militants, local elders and civilians leaving the governor's compound after attending a special event for Eid.
On Saturday a suicide assault on a gathering of Taliban, security forces and civilians in the province killed at least 36 people and wounded 65, Kamawal said.
Taliban spokesman Zabihullah Mujahid announced the decision to resume fighting after President Ashraf Ghani said Saturday a government truce with the militants would be extended. He asked the group to reciprocate.
"The cease-fire ends tonight and our operations will begin. We have no intention to extend the cease-fire," Mujahid said.
Mujahid made no reference to Ghani's announcement.
The first formal nationwide cease-fire since the 2001 US invasion had been widely welcomed across the country as Afghans – Taliban, security forces and civilians – celebrated Eid, the holiday that caps the fasting month of Ramadan.
Taliban militants and security forces embraced and took selfies with each other over the first two days of the Muslim holiday.
Civilians also flocked to greet the militants, who had left their posts or areas under their control to celebrate the halt in hostilities, fueling hopes among war-weary Afghans that peace was possible.
Ghani's extension of the government's eight-day cease-fire, which had been due to expire Tuesday night, drew immediate international support and calls for the Taliban to follow suit.
The Taliban had agreed to a truce but only for the first three days of Eid, which started Friday, promising not to attack Afghan soldiers or police. They would, however, continue attacking US-led NATO troops.
With the cease-fire due to end Sunday night, fighting is likely to resume on Monday.