Habib Shah Ansari, head of the health department in Khost, a mountainous region on the border with Pakistan, said at least 34 were injured in the blast.

There was no immediate claim of responsibility for the explosion, which adds to a series of attacks on voter centres since the registration process began in April. The Taliban, which has warned people against taking part in the election process, issued a statement denying any involvement.

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Medical aid group Medecins Sans Frontieres said it had donated emergency supplies to the Khost provincial hospital where many of the wounded were taken.

Last month, around 60 people were killed in a suicide attack on a voter centre in the capital Kabul, which was claimed by Islamic State and there have been a string of smaller incidents that have deterred many from going to the voter centres.

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 travelling in hit a roadside bomb early on Sunday morning, Mohammad Karim Yuresh, spokesman for Faryab police, said.

Reuters