Khost provincial governor spokesman Taliban Mangal said at least nine people had been killed and 22 wounded, but that could not be immediately verified by military officials.
There was no immediate claim of responsibility for the blast in the eastern province, which defence ministry spokesman Ghafoor Ahmad Jawed would only confirm had caused casualties.
Some of the wounded had been transported to a hospital in the provincial capital Khost while four helicopters also had been dispatched to bring others to Kabul, Jawed said.
It is not clear how many worshippers were inside the mosque at the time of the explosion or the nature of the blast.
Khost is located on the border with Pakistan’s tribal areas.