Clashes in southern Afghanistan between Taliban and government forces on Sunday marked the end of a three-day truce for Eid Al Fitr.
The truce was marred by several attacks claimed by ISIS.
Taliban insurgents and government officials accused each other of resuming hostilities after a rare respite from increasing violence in the country, as US and other foreign forces prepare to withdraw later this year.
Attaullah Afghan, head of the Helmand provincial council, said Taliban fighters attacked security checkpoints on the outskirts of Lashkar Gah, the capital of the restive province, and some other districts.
"The fighting started early today and is still ongoing," he told AFP.
An Afghan army spokesman confirmed that fighting had resumed, and the Helmand governor's office said that 21 Taliban fighters had been killed.
The Taliban denied that it resumed fighting first.
"They [Afghan forces] started the operation … do not put the blame on us," Taliban spokesman Zabihullah Mujahid told AFP.
The three-day truce initiated by the Taliban and swiftly agreed to by the Afghan government had largely held during the Eid holiday that ended on Saturday.
However, the calm was shattered by a bombing on Friday in a mosque north of the capital. This killed 12 people, including the imam leading the Friday prayers, and injured 15.
The Taliban denied involvement and blamed the government intelligence agency.
But on Sunday, the Afghan affiliate of ISIS claimed the attack, saying its fighters planted an explosive device in "a worship place for disbeliever Sufis", killing the "apostate imam".
The extremists also claimed responsibility for explosions at several electrical grid stations that left the capital Kabul without power for much of the three-day holiday.
In posts on its affiliated websites, ISIS claimed other attacks over the past two weeks that destroyed 13 electrical grid stations in several provinces. The stations bring imported power from Uzbekistan and Tajikistan.
Attacks disrupted power supply in nine provinces, including Kabul, said Sanger Niazai, a government spokesman.
The Eid truce was only the fourth agreed pause in fighting in the two decades of conflict since a US-led invasion toppled the Taliban regime.
The insurgents began their first direct peace talks with the government last year, as part of a deal with the US that includes the withdrawal of American forces from Afghanistan.
Taliban political spokesman Suhail Shaheen said negotiating teams met briefly on Saturday in Qatar, which is hosting the talks.
They renewed their commitment to finding a peaceful end to the war and called for an early start to talks that have been stalled, he said.
Violence has soared in Afghanistan since the US and Taliban signed their agreement in February last year.
The deal called for all US troops to leave by May 1 this year but President Joe Biden, who took office in January, pushed the deadline back to September 11 – exactly two decades on from the Al Qaeda terrorist attacks in the United States, which led Washington to invade Afghanistan.
Tens of thousands of Afghans have been killed and millions displaced by the conflict, in which a resurgent Taliban have taken hold of large parts of the country.
Nishank Motwani, an independent Afghanistan expert based in Australia, told AFP the Taliban viewed the American withdrawal as a triumph.
"It gives the insurgents a proclamation of victory, bookends their removal and eventual return to power, and signals that the end is in sight for the Afghan republic in its current state," he said.
Government forces have continued to receive vital air support from US warplanes, and there are concerns over whether they would be able to hold back the insurgents without Washington's help.
"It is now going to be very difficult for us to conduct operations," an Afghan army officer told AFP last week after US forces pulled out fully from Kandahar Airfield, once the second-largest base of coalition forces.
"Our aircraft can't fly at night, so the night operations are going to be difficult."