Commissioner Kohat Khalid Omazai told Arab News the toll could go up as some people were trapped under the debris.
Officials said Shiites were praying in a room inside the hospital when the bomb exploded in the town of Hangu.
Several police officials said the bomber drove the tractor into the hospital in Hangu. They said three policemen were among the dead. Sixteen people were wounded.
Authorities have beefed up security to avert sectarian violence during Muharram.
There have been several sectarian attacks and clashes in Hangu in recent years.
On Wednesday, a suicide bomber blew himself up beside a minibus in the nearby town of Kohat, killing at least 16 people, officials said.
The Pakistani Taleban movement, which is linked to Al-Qaeda, claimed responsibility for a twin suicide bombing on the office compound of a government official in the Mohmand region of the northwest on Monday, which killed at least 40 people.
— With input from agencies
