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 Id ul-Fitr holiday during a ceasefire.

The Taliban insurgent group, which declared the three-day truce for the holiday, condemned the attack in a statement, and there was no immediate claim of responsibility.

Ferdous Faramarz, spokesman for the Kabul police, said the mosque’s Imam was among the 12 dead and that at least 15 people were wounded in the blast at a mosque 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 Shia minority. The Taliban also denounced that attack and no one claimed responsibility.

US 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 US has begun an operation to withdraw all its remaining troops.