The blast took place in the Darul Aman area of the city, local media reported. 1TV channel reported that the blast took place during a Shiite ceremony in Baqer-ul-Oloom mosque, west of Kabul.
Children were among those killed in the attack, TOLOnews reported.
The suicide bomber reportedly detonated his explosive vest after entering the mosque.
Taliban spokesman Zabihullah Mujahid has denied his group had a role in the fatal attack, the network added.
This was the second blast to rock Kabul on Monday.
Earlier in the day two people were injured in an explosion in Bagrami district, Basir Mujahid, a Kabul police spokesman, confirmed to Pajhwok.