An explosion inside a mosque on the outskirts of the Afghan capital Kabul killed at least 12 people during Friday prayers as worshippers gathered for the Eid al-Fitr holiday during a ceasefire.

The Taliban insurgent group, which declared the three-day truce for the holiday, condemned the attack in a statement. Islamic State claimed responsibility for the attack on Saturday through its Nasheer news agency on Telegram.

Ferdous Faramarz, the spokesman for the Kabul police, said the mosque's Imam was among the 12 dead and at least 15 people were wounded in the blast in the capital's Shakar Dara district. The blast came less than a week after an explosion at a school killed 80 people, most of them schoolgirls from the ethnic Hazara Shi'ite Muslim minority. The Taliban also denounced that attack and no one claimed responsibility.

U.S. officials believe the attack on the school may have been the work of a rival militant group such as the Islamic State. Such groups have not signed on to the holiday ceasefire. Violence, including attacks on civilians, has increased in Afghanistan even as the United States has begun an operation to withdraw all its remaining troops over the next four months.

There have been no major reports so far of direct fighting between government forces and the Taliban during the holiday ceasefire, which began on Thursday at the end of the Ramadan fasting month. However, roadside bombings have continued with at least 11 civilians reported killed and 13 wounded in four bombings on Thursday. The government and the Taliban have been holding political talks to try to end their conflict as Washington pulls its troops out 20 years after US bombing forced the Taliban from power. Both sides at the talks have accused the other of provoking and failing to halt attacks against civilians.

"Today's attack on a mosque in Shakar Dara district of Kabul during Friday prayer is completely opposing the idea of Eid al-Fitr as a family holiday celebrated in peace," the European Union's mission in Afghanistan said on Twitter.