Police prevented the bomber from entering the building, but he blew himself up at the entrance killing himself and five other people.
Another 13 people were injured.
The officials blamed Saturday's attack on the Allied Democratic Forces (ADF), a militant group said to be linked to the so-called Islamic State (IS).
So far no group has claimed responsibility for the attack.
More than 30 people were celebrating Christmas at the In Box restaurant when the bomb went off, two witnesses told AFP news agency.
