The Taliban refused to extend their ceasefire beyond Sunday night, dampening hopes for peace after jubilant scenes over the Eid holidays in Afghanistan.
The announcement came after a suicide attack in the restive eastern part of the country on Sunday killed at least 18 people in a crowd celebrating the Muslim holiday, the second assault in as many days to mar the unprecedented ceasefire.
Kabul extended its ceasefire with the Taliban by 10 days but said security forces would defend themselves if attacked, a spokesman for President Ashraf Ghani tweeted.
The Afghan leader also requested the militant group halt hostilities but the Taliban said fighting would resume.
"The ceasefire ends tonight and our operations will begin, inshallah (God willing). We have no intention to extend the ceasefire," Zabihullah Mujahid, the group's spokesman, told AFP in a WhatsApp message.
He made no reference to Ghani's announcement.
The announcement has raised concern among some Afghans over the number of Taliban who have taken advantage of the ceasefire to enter cities around the country, including the capital Kabul, and may still be there when the truce ends.
The Islamic State group, which was not part of the truce, claimed it had carried out its second suicide attack in two days in the province of Nangarhar.
Provincial health director Najibullah Kamawal put the toll from Sunday's blast in Jalalabad city, outside the office of the Nangarhar provincial governor, at 18 dead and 49 wounded.
"Some of the wounded are in a serious condition," Kamawal added, suggesting the death toll could rise.
The governor's spokesman, Attaullah Khogyani, put the death toll slightly higher at 19.
He said a bomber on foot blew himself up among a crowd of Taliban fighters, 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 told AFP.
The Islamic State's Afghanistan franchise, which is particularly active in the east, claimed responsibility for that attack.
The group has killed hundreds of people in multiple attacks across the country since it first emerged in the region in 2014. It has also fought the Taliban in some areas.
- No surprises -
The first formal nationwide ceasefire 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 fighters 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, fuelling hopes among war-weary Afghans that peace was possible.
Ghani's extension of the government's eight-day ceasefire, 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.
Adding to unease among ordinary Afghans, who have borne the brunt of the nearly 17-year war, is the number of Taliban fighters now inside cities around the country.
"Seeing all the strange face(s) on the streets of Kabul, I am concerned. I hope there is not deception at work," Mohammad Saber wrote on Facebook.
A user who went by the name Kargar posted: "There is word that hundreds of Taliban fighters have come to Kabul, but only a small number have left. Can Kabul police guarantee our security?"
Before the Taliban's ceasefire had even started, analysts had expressed cautious optimism that the truces, if successful, could help build trust between the government and the Taliban and lay the groundwork for peace talks.
But it was clear that not everyone in the Taliban approved of the bonhomie between their fighters and security forces.
Following Saturday's attack, the Taliban ordered their fighters to avoid gatherings of security forces and civilians, ostensibly to avoid further civilian casualties.
"The enemy has misused the ceasefire issue and there is a chance of more such bad incidents happening," the group's spokesman Mujahid said in an earlier message.
But some Taliban commanders also told AFP they disapproved of their fighters visiting government-controlled areas and celebrating with police and troops.
The Taliban's decision to resume fighting came as no surprise to several Western diplomats in Kabul.
"If they extend the ceasefire they will be compelled to talk, which I think the Taliban isn't interested in. They're looking for an outright victory," one diplomat told AFP on condition of anonymity.