An explosion at a mosque in the eastern Afghan province of Khost on Sunday killed at least 13 people and wounded 31 and the casualty total may rise, a local police official said. Basir Bina, spokesman for the provincial police, said people were gathered after afternoon prayers in the mosque, which was also being used as a voter registration center for parliamentary elections due in October. He said the blast appeared to have been caused by explosives left in the mosque rather than by a suicide bomber. The bomb had been placed in a tent being used to register voters on the grounds of a mosque, Khost provincial police chief Abdul Hanan Zadran told AFP. "A crowd of people who had come out of the mosque had gathered to register" when the blast took place, he added. There was no immediate claim of responsibility for the explosion, which adds to a series of attacks on preparations for elections seen as a key test of government credibility. However, both the Taliban and a local ISIS affiliate reject democratic elections and have targeted them in the past. ISIS is not known to have a presence in Khost, but has expanded its footprint into other areas in recent years. Last month, around 60 people were killed in a suicide attack on a voter center in the capital Kabul, which was claimed by Islamic State while the Taliban has also warned people not to take part in the elections. Sunday’s blast came as fighting has spread across Afghanistan, with government officials saying that a district in the northern province of Badakhshan which was seized by Taliban fighters last week had been retaken. However Taliban spokesman Zabihullah Mujahid denied the reports that Kohistan district had been retaken, saying in a statement that security forces had been driven back. In a separate incident in the northern province of Faryab, seven people were killed when the car they were traveling in hit a roadside bomb early on Sunday morning, Mohammad Karim Yuresh, spokesman for Faryab police, said. The Taliban and ISIS have launched a relentless wave of attacks since the start of the year, killing scores of civilians in the capital, Kabul, and elsewhere. Afghan security forces have struggled to combat the groups since the US and NATO concluded their combat mission at the end of 2014, switching to a counterterrorism and support role.