At least 17 people have been killed and 37 wounded in an explosion at a mosque in the Afghan province of Khost, BBC cited local officials as saying.
People had gathered for afternoon prayers at the mosque, which was also being used as a voter registration centre.
Some of the injured are said to be in a critical condition.
No group has claimed responsibility but ISIS group has carried out similar attacks in the past.
The latest incident appears to have been caused by explosives left in the mosque, rather than by a suicide bomber.
There have been a number of attacks on voter registration centres since the process started last month for October's parliamentary elections.
On 22 April, a suicide bomb attack at a voter registration centre in the capital Kabul killed at least 57 people.