Security forces keep watch at the site of a blast in Kabul, Afghanistan. (Reuters)

A blast in the centre of Afghanistan's capital Kabul killed at least six people on Tuesday, police officials said.

No group immediately claimed responsibility for the attack.

"We are investigating whether it was a suicide attack or whether a magnetic bomb was attached to a motor-bike," interior ministry spokesman Nasrat Rahimi said.

In a separate incident, an explosion near an election rally attended by Afghan President Ashraf Ghani killed 24 people and injured 31 others in Charikar, capital of Parwan province, north of Kabul, health officials said.

Taliban commanders have said they will intensify clashes with Afghan and foreign forces to dissuade people from voting in the Sept. 28 presidential election, when Ghani will bid for a second five-year term.